Operation Ai


This was the Japanese carrierborne air attack on the US Pacific Fleet at Pearl Harbor in the Hawaiian islands group, planned as ‘Z’ (i) and through its implementation precipitating the USA’s entry into World War II (26 November/7 December 1941).

The Japanese navy planned and implemented ‘Ai’ in an effort to destroy Admiral Husband E. Kimmel’s US Pacific Fleet at its main Pacific base on the island of Oahu in the Hawaiian islands group, and also to inflict significant damage on major US Army, US Marine Corps and US Navy aircraft and installations on the same island. The attack secured total surprise at the strategic, operational and tactical levels, and was operationally and tactically successful inasmuch as it achieved the sinking of seven US ships and the damaging of another 11, the destruction of 188 aircraft and the damaging of another 159, and the killing or wounding of large numbers of naval and military personnel as well as comparatively small numbers of civilians to the extent of 2,402 dead and 1,247 wounded.

Admiral Isoroku Yamamoto, commanding the Combined Fleet, was basically opposed to the Japanese plan for territorial expansion if this involved war with the more populous, resources-rich and industrialised USA but insisted, should Japan go to war, that Pearl Harbor and the US Pacific Fleet be added to the list of initial Japanese objectives in the first days of Japan’s war. Yamamoto appreciated that this pre-emptive blow would provide the Japanese with vital security as they expanded the perimeter of their empire.

Though planned under Yamamoto’s direct supervision, ‘Ai’ was led by Vice Admiral Chuichi Nagumo, whose force lost only 29 aircraft and 64 men. But the most important warships of the Pacific Fleet, namely its three fleet carriers, were not in port during the attack and so escaped damage or destruction, as did the base’s vital oil tank farms, submarine pens, machine shops and other installations vital to the US Navy’s short- and medium-term prosecution of the war against Japan: with these resources, therefore, the USA was able to rebound to an offensive capability in little more than six months.

The US public saw the attack as a treacherous act, moreover, and ‘Ai’ therefore served to rally the whole American population against Japan as the USA entered World War II.

The Japanese had seized Manchuria in 1931 and turned it into the puppet kingdom of Manchukuo, and had been fighting the 2nd Sino-Japanese War with China since 1937. These operations on the Asian mainland stemmed largely from the almost total independence, from the control of both the Japanese political leadership and military high command, of the Imperial Japanese army’s formations in the areas, and their desire to implement Japan’s search for economic self-sufficiency by an expansion to the north, north-west and west against the USSR, Mongolia and China. At the same time the Imperial Japanese navy’s desire to achieve the same self-sufficiency in resources (especially minerals, oil, rubber and food) was focused on the so-called ‘Southern Resources Area’ represented by the US, British, Dutch and French possessions in South-East Asia and in the seas off them.

The rebuff of the Imperial Japanese army in its various encounters with the USSR on the Manchukuoan/Siberian frontier in 1938/39 combined with the signature of the August 1939 non-aggression pact between Germany and the USSR to persuade the service that there was greater short-term profit in the navy’s concept of expansion to the south rather than the north, and planning for such an expansion of Japan was therefore started, becoming the first priority at the end of June 1940, which was the month in which Germany sealed the military defeat of France.

However, Japan’s military adventures in Asia, most importantly the war with China, were by 1941 further exacerbating the existing tensions between Japan on the one hand and the USA and UK on the other. The two major western powers reacted to Japanese military action in China by halting the export to Japan first of scrap metal and second of oil, thereby crippling Japanese heavy industry and transport, then froze Japanese assets and closed the Panama Canal to Japanese shipping. Diplomatic negotiations climaxed with a note of Secretary of State Cordell Hull on 26 November 1941, which the Japanese prime minister, General Hideki Tojo, described to his cabinet as an ultimatum. The oil boycott which had started during the summer of that year was especially threatening to Japan in general and to the Japanese navy in particular as Japan lacked any oil resources of her own. The Japanese leadership now appreciated that only two choices were available to it: either to accede to the US and British demands by initiating a withdrawal from China, or to escalate the conflict in an effort to acquire control of oil and other vital raw materials, primarily in South-East Asia.

By the time of the Hull note, the Japanese authorities had opted for the second course. This was based on the rapid seizure, by a force totalling of 11 army divisions and supported by a number of special naval landing forces together with army and navy air formations, of British interests in the form of Hong Kong, Malaya, the smaller part of Borneo and Burma, Dutch interests in the form of the East Indies and the larger part of Borneo, French interests in the form of Indo-China and, at the insistence of the army, US interests in the form of the Philippine islands group. It is worth noting that the most important target region was the East Indies, which had the huge quantities of many of the raw materials Japan needed, while the areas flanking this drive to the south had also to be taken not only for the raw materials they too offered, but also to protect the eastern and western flanks of the primary expansion to the south. The rapid conquest of these areas would be undertaken by the army with strong naval support while the navy prevented any US interference from bases on the US western seaboard and in the Hawaiian islands group.

Given the fact that the Japanese had now decided to seize the ‘Southern Resources Area’, Yamamoto demanded that the Pacific Fleet be neutralised in Pearl Harbor to provide the Japanese with the time to create a large defensive perimeter to provide the time and space that would shield the ‘Southern Resources Area’ from any short-term US response. In this the Japanese rightly estimated that the USA’s strategic plan was for the retention of the Philippine islands group against Japanese aggression for a time long enough for major reinforcements to arrive from the US west coast via Pearl Harbor, with the islands of Wake and Guam serving an staging points on the long westward line of advance toward the Philippine islands group.

The key decision was therefore to undertake the defeat of the Pacific Fleet in Pearl Harbor even as army forces swept into the ‘Southern Resources Area’ of what had been announced in 1940 as the ‘Greater East Asia Co-Prosperity Sphere’ but had first been promulgated in 1938 as the ‘New Order in East Asia’, and then the swift reduction of Wake and Guam islands to prevent their use as staging posts in any US advance across the Pacific. Any such US advance would therefore have to make the extraordinarily difficult and lengthy strategic and operational ‘leap’ from the Hawaiian islands group westward to the Philippine islands group in a single bound with wholly inadequate naval support. This, the Japanese reckoned, might bring the Americans to the negotiating table to agree a peace on the basis of the status quo, or alternatively lay the Americans open to defeat in detail as they attempted the trans-Pacific leap to the Philippine islands group.

The formal decision to add Pearl Harbor to the list of objectives in Japan’s initial strategic offensive was taken as late as 10 November, but the Imperial Japanese navy’s contingency planning for this eventuality had started at somewhat earlier date. So far as the ‘Z’ (i) planning was concerned, the Japanese navy had been impressed with the ‘Judgement’ (i) operation of Admiral Sir Andrew Cunningham’s Mediterranean Fleet, in which 20 obsolescent Fairey Swordfish biplanes launched from a fleet carrier had disabled half the Italian battle fleet in Taranto harbour and forced the withdrawal of the Italian fleet to ports to the north of Naples. Yamamoto dispatched a naval delegation to Italy, a Japanese ally, and this concluded a larger and better supported version of the ‘Judgement’ (i) concept could cripple the Pacific Fleet at Pearl Harbor and force its surviving ships back to the safer ports of the west coast of the continental USA, so giving Japan the time and opportunity to establish and secure the ‘Greater East Asia Co-Prosperity Sphere’, the euphemism denoting a Japanese seizure of the resources-rich areas of South-East Asia and the oil reserves of the Netherlands East Indies (the ‘Southern Resources Area’), and the creation of a powerful defensive perimeter to shield Japan and her new acquisitions.

Most importantly, the Japanese delegation returned from Italy to Japan with the concept of the shallow-running torpedo which Cunningham’s technical staff had devised. Additionally, some Japanese strategists may have been influenced by the actions of Vice Admiral Harry E. Yarnell in the 1932 joint US Army and US Navy exercises, which presumed an invasion of Hawaii by an enemy. In the role of the commander of the attacking fleet, Yarnell sailed his aircraft carriers to the north-west of Oahu into rough weather, and launched attack aircraft on the morning of 7 February 1932. The exercise’s assessment team recorded that Yarnell’s aircraft were able to inflict serious damage on the defenders, who were still unable to locate his fleet as late as 24 hours after the attack. The US Navy’s conventional doctrine of the time believed that any attacking force would be found and destroyed by the battleship force stationed at Pearl Harbor, however, and thus dismissed Yarnell’s strategy as lacking any real-world practicality.

In 1941 US officials claimed that Pearl Harbor was the most strongly defended fortress in the world. The falsity of that claim became evident on 7 December 1941, but what cannot be denied is that Pearl Harbor was well defended against what was, at the time, deemed the only practical form of attack. The coastal artillery around the harbour was impressive, and included six batteries of battleship-calibre guns, eight batteries of cruiser-calibre guns, and 12 smaller batteries along with 26 fixed 3-in (76.2-mm) anti-aircraft guns, 60 mobile 3-in (76.2-mm) anti-aircraft guns, 20 37-mm anti-aircraft guns, and 107 0.5-in (12.7-mm) anti-aircraft machine guns. These were all under control of the US Army, with headquarters at Fort Kamehameha, and were manned by the 15th, 16th and 251st Coastal Artillery Regiments. The largest guns could be directed against any point on the Oahu coast. However, the guns were not manned on 7 December, a Sunday, and fired not a single shot during the Japanese attack.

The Oahu garrison consisted of about 25,000 men of Major General Durward S. Wilson’s 24th Division and Major General Maxwell Murray’s 25th Division, which were as well trained at that time as any in the US Army. Major General Frederick L. Martin’s Hawaiian Air Force boasted 154 fighters (although only 99 were of the most modern available type, the Curtiss P-40) and 57 bombers (of which 24 were modern Boeing B-17 and Douglas A-20 machines). On the island there were also five newly installed radar stations with the SCR-270B equipment, but the associated fighter control centre was not yet fully operational.

A conventional attack from the sea could in fact have faced a very severe handling.

The harbour itself was not considered for development as a major base until the 1930s, though a small naval station had been established as early as 1901. Geographically, Pearl Harbor is actually a drowned river delta within a barrier reef, shallow and with a single narrow entrance. While the latter had some advantages for anti-submarine defence, its narrow mouth also meant that it could take some hours for the Pacific Fleet to sortie. By 1941, the harbour had extensive facilities, including dry docks, machine shops, and oil storage equal to that of the entire Japanese empire (563,000 tons in 54 tanks), none of which were damaged in the attack. The shipyard alone had an area of 498 acres. The harbour was still in the process of being dredged, but there was a deep-water anchorage sufficient for 100 warships, so long as there was no objection to anchoring them in close clusters.

The Japanese rationale of the attack on Pearl Harbor was based on the desire to neutralise, on at least a temporary basis, US naval power in the Pacific as part of a series of nearly simultaneous and neatly co-ordinated attacks right across the Pacific theatre against targets in several countries. Though not convinced that Japan could survive the inevitable counter from the Americans and British, Yamamoto did believe that a successful attack would buy Japan a year or perhaps slightly more in which to exploit the freedom of action resulting from a successful attack.

The Pearl Harbor operation was conceived by Yamamoto in his capacity as commander-in-chief of the Combined Fleet, which included most of the major warships of the Japanese navy. It is not known when Yamamoto first developed the idea, but his friend Rear Admiral Shigeru Fukudome, then the chief-of-staff of the Combined Fleet, first heard Yamamoto suggest an attack on Pearl Harbor in March or April 1940. Though opposed to a war against the USA, Yamamoto felt that if war was unavoidable then Japan’s only hope was a devastating surprise attack on the Pacific Fleet at the moment war broke out, which would allow Japan to seize and consolidate the resource-rich areas of South-East Asia and the South-West Pacific and so be in a position to force the Allies to accept a negotiated peace.

The notion of a surprise attack on Pearl Harbor at the start of a war against the USA was not new, for it had been war-gamed as early as 1927 by the Japanese Naval Staff College. Nevertheless, Yamamoto’s concept was highly unorthodox. Japanese naval doctrine had long focused on a 'great decisive battle' to be fought between US and Japanese battleships somewhere close to Japan, in the manner of the decisive Battle of Tsushima of the 1905 Russo-Japanese War. As a fleet commander, Yamamoto was supposed to conduct training and carry out operations planned by the Navy General Staff, but nonetheless persevered with conceptual development of a attack on Pearl Harbor. By December 1940, following the annual fleet manoeuvres, Yamamoto had decided that the operation must become part of the war plan.

Detailed planning was left to Commander Minoru Genda, the very talented air officer of the 1st Carrier Division. Genda immediately saw the possibilities of the concept, and also the challengers inherent in it, and pressed for the Pacific Fleet’s carriers and cruisers to be designated as the priority targets. Genda also wished to make Oahu the principle target of the Japanese opening offensive, in order to deprive the Americans of their most valuable Pacific base. However, his proposal for an amphibious assault on the island was rejected at once by Rear Admiral Takajiro Onishi, the chief-of-staff of the 11th Air Fleet controlling the majority of the Japanese navy’s land-based aircraft, and certainly would not have had the support of the Japanese army. Genda had completed a draft operational plan by February 1941.

After the idea of seizing Oahu had been rejected, Genda put no further emphasis on the destruction of Pearl Harbor’s base facilities. The target was the Pacific Fleet and its supporting air power. It is also worth noting that Yamamoto did envisage Pearl Harbor itself as a target: his real object was the destruction of US public opinion and its faith in the US armed forces. By the destruction of the Pacific Fleet’s battleships, which were to US public opinion the very symbol of US naval power, Yamamoto hoped to strike such a devastating blow against American morale that the pacifist and isolationist elements of the US public would demand a negotiated settlement favourable to Japan. To achieve this, he was willing to risk heavy losses to his carrier fleet.

Inevitably, of course, surprise and secrecy were considered vital to the success of the attack, but there were leaks that might have proven disastrous for the Japanese had the Americans not discounted them. On 27 January 1941, for example, the US ambassador to Japan, Joseph Grew, sent a telegram to the Department of State reporting that he had received a report from the Peruvian ambassador that the Japanese planned to attack Pearl Harbor in the event of war. Grew considered the report fantastic, and so did US Navy intelligence, and nothing further was done to investigate the matter further. The Peruvian ambassador seems to have obtained the information from his Japanese cook, and it is unclear how the cook came by the 'information': the idea of an attack on Pearl Harbor had been a hackneyed concept of Japanese writers for years before the war, and the cook may simply have been repeating the cliché.

The Japanese plan initially called for an attack by a force of three fleet carriers. Following table-top manoeuvres on 9 October 1941, Rear Admiral Tamon Yamaguchi, commander of the 2nd Carrier Division, Rear Admiral Ryunosuke Kusaka, chief-of-staff of the 1st Air Fleet and Genda all became convinced that all six of Japan’s fleet carriers should be committed to the operation. The three officers persuaded Nagumo, the 1st Air Fleet's commander, to send Kusaka to ask the naval chief-of-staff, Admiral Osami Nagano, to approve a six-carrier undertaking. Nagano gave his approval on 18 October after discussions with Kusaka. The six carriers would be able to launch two attack waves totalling more than 300 aircraft against Pearl Harbor while maintaining a strong combat air patrol over their own precious selves.

The tactical element of the attack was based on the use of the torpedo as the primary weapon type, but the air-launched torpedo of the time required comparatively deep water to become stable in the water without striking the bottom. During the summer of 1941, therefore, Japan secretly created and tested modifications, including wooden fins, which would allow an air-launched torpedo to work properly after release into shallow water such as that of Pearl Harbor. The effort resulted in the Type 91 torpedo, the weapon which inflicted most of the damage to US ships during the attack.

Tests also suggested that a 1,764-lb (800-kg) bomb would penetrate battleship deck armour if dropped from an altitude of 12,000 ft (3660 m). The Japanese were short of the special steel required for such weapons, but found they could convert 16-in (406-mm) armour-piercing shells to make effective bombs. Designated the Type 99 No. 80-3, the converted bombs were streamlined to improve penetration and carried 50 lb (22.8 kg) of high explosive. Production was slow, however, and only 150 of the bombs had been completed by mid-September 1941, while the last of the specially modified torpedoes were finished barely in time to be rushed to the task force rendezvous by the carrier Kaga.

Aircrew training for the mission was exhaustive. The first attack wave included a total of 89 Nakajima B5N 'Kate' torpedo bombers, of which 49 were configured as horizontal bombers, operating from Akagi, Kaga, Hiryu and Soryu; 51 Aichi D3A 'Val' dive-bombers operating from Shokaku and Zuikaku; and 45 Mitsubishi A6M Reisen 'Zero' fighters operating from all six carriers. The fighters and dive-bombers were to attack Oahu’s airfields while the torpedo and horizontal bombers were to concentrate their efforts on the carriers and battleships. The second attack wave comprised 54 B5N machines from Shokaku and Zuikaku configured as horizontal bombers and 80 D3A and 36 A6M warplanes from the other four carriers. The inexperienced torpedo bomber crews of Shokaku and Zuikaku, which had only recently been commissioned, were given the relatively easy task of bombing the airfields, while the other warplanes of the second wave were to finish any damaged warships. The attack waves were to concentrate their efforts on destroying the largest warships and not to spread their attacks, since moderately damaged vessels would be relatively easy for the Americans to salvage, and the dive-bombers of the second attack wave had specific orders to wreck the exposed hulls of any capsized carriers.

There were disagreements at different command levels on what should be ordained as the priority targets for the raid. The formal order from the naval general staff designated US land-based air power as the first priority, followed by the carriers and then the battleships. In all probability this reflected the naval general staff’s fears that Nagumo’s force would be highly vulnerable to a US counterattack. Yamamoto ignored the naval general staff’s instruction in his own order to Nagumo, which gave the first priority to the disabling of four battleships and the second priority to the disabling of four carriers. Genda felt strongly that the carriers should be the priority target, however, and there are clear indications that his thinking strongly influenced how the operation was finally planned and executed.

As noted above, the carrier Kaga arrived in Saeki Bay on the island of Kyushu to load the 100 special torpedoes needed for ‘Ai’. On 18 November the Imperial Japanese navy began to assemble the support ships, most especially the oilers Kenyo Maru, Kokuyo Maru, Kyokuto Maru, Toei Maru, Toho Maru, Nippon Maru and Shinkoku Maru, which were also a vital element of ‘Ai’.

On 19 November the fleet carriers Shokaku and Zuikaku departed the Inland Sea for Hitokappu Bay in the Kurile islands group to join the ships massing for ‘Ai’, for which Shokaku’s and Zuikaku’s embarked squadrons had been increased beyond the standard 18 by the addition of aircraft and aircrews assigned from shore-based training squadrons: Zuikaku thus carried 18 A6M fighters, 27 D3A dive-bombers and 27 B5N level and torpedo bombers. Finally, an additional 12 spare aircraft of each type were embarked in a disassembled condition.

On 26 November 1941, Nagumo’s Pearl Harbor Strike Force, based on the six fleet carriers of the 1st Air Fleet, departed Hitokappu Bay on the eastern side of Etorofu island, under conditions of great secrecy including strict radio silence, and headed for the Hawaiian islands group via a circuitous route through the inhospitable and thus only sparsely used northern Pacific to reduce the chances of the force’s detection. The carriers were a trio of sister or half-sister ships, namely Akagi and Kaga, Hiryu and Soryu, and Shokaku and Zuikaku (Nagumo’s 1st Carrier Division, Rear Admiral Tamon Yamaguchi’s 2nd Carrier Division and Rear Admiral Chuichi Hara’s 5th Carrier Division respectively). The fast battleships Hiei and Kirishima (Vice Admiral Gunichi Mikawa’s 3rd Battleship Division), heavy cruisers Tone and Chikuma (Rear Admiral Hiroaki Abe’s 8th Cruiser Division), and light cruiser Abukuma and destroyers Isokaze, Urakaze, Tanikaze, Hamakaze, Arare, Kasumi, Kagero, Shiranuhi and Akigumo (Rear Admiral Omori Sentaro’s 1st Destroyer Squadron) escorted the task force, and the fleet submarines I-19, I-21 and I-23 (Captain Kijiro Imaizumi’s 2nd Submarine Division) provided shipping lane reconnaissance. There were also eight oilers and supply ships (the 1st Supply Train comprising Kyokuto Maru, Kenyo Maru, Kokuyo Maru, Shinkiku Maru and Akebono Maru, and the 2nd Supply Train comprising Toho Maru, Toei Maru and Nippon Maru) as the main force’s fleet train, and the destroyers Sazanami and Ushio for detachment to undertake a gunfire bombardment of Midway island as Captain Kaname Ohishi’s Midway Destruction Force.

The carriers embarked 414 aircraft of just three types, namely the A6M fighter, B5N torpedo and level bomber, and D3A dive-bomber.

Refuelling from the eight oilers and supply ships was undertaken on this outward leg of the operation was accomplished on 4 December just to the west of the international date line in the area to the south of the Aleutian islands group.

The operation was also supported by the Advanced Expeditionary Force (Vice Admiral Mitsumi Shimizu’s 6th Fleet), which included 20 fleet submarines (Rear Admiral Tsumoto Sato’s 1st Submarine Squadron with I-9, I-15, I-17 and I-25, Rear Admiral Shigeaki Yamazaki’s 2nd Submarine Squadron with I-1, I-2, I-3, I-4, I-5, I-6 and I-7, and Rear Admiral Shigeyoshi Miwa’s 3rd Submarine Squadron with I-8, I-68, I-69, I-70, I-71, I-72, I-73, I-74 and I-75), and Lieutenant Iwasa Naoji’s force of five two-man midget submarines (I-16A, I-18A, I-20A, I-22A and I-24A) carried by I-16, I-18, I-20, I-22 and I-24 of Captain Hanku Sasaki’s Special Attack Unit, supported by I-10 andI-26 (Commander Yasuchika Kashihara’s Submarine Reconnaissance Unit). These boats were intended first to gather tactical intelligence round the Hawaiian islands group and second to sink any US vessels that might try to escape from Pearl Harbor during the Japanese attack.

Pearl Harbor Naval Base was under the command of Vice Admiral Claude C. Bloch and the army garrison under the command of Lieutenant General Walter C. Short. The latter was well regarded, but appears to have been confused about his task. His force was in Hawaii to protect the Pacific Fleet at its base, but Short seems to have thought that the Pacific Fleet was there to protect the Hawaiian islands group. Short also was obsessed with the idea of sabotage and less concerned with the possibility of an air raid. His orders to park the aircraft in neat rows in the centre of the runways, rather than have them dispersed in their revetments where more manpower was required to guard them, left the aircraft highly vulnerable to air attack. However, the entire US high command was obsessed with sabotage as it was generally believed that 'fifth columnists' had been active during the early German victories in Europe.

Kimmel had organised his Pacific Fleet into three task forces, but was unable to keep more than one task force at sea at any given time for lack of adequate numbers of oilers: the Pacific Fleet had only 11 of these all-important support vessels, of which just four were equipped for under-way replenishment, as a result of the transfer of support ships to the Atlantic.

On the morning of the 'Ai' attack, Vice Admiral William F. Halsey, the commander, Carriers, Pacific Fleet, was returning from an aircraft ferry mission to Wake island with approximately half his two-carrier force, while the other half was preparing to fly off aircraft for the reinforcement of Midway island. The Pacific Fleet’s third carrier was at San Diego for a refit and, as a result, all three carriers of the Pacific Fleet were absent from Pearl Harbor and were therefore spared the attack. However, Vice Admiral Wilson Brown, the commander, Scouting Force, Pacific Fleet, was leading a small exercise of Johnston island, leaving most of his force and almost all of Vice Admiral William S. Pye’s Battle Force, Pacific Fleet, at Pearl Harbor.

The 'Ai' attack caught the US Army at a particularly unfortunate moment. Short had had his troops at a high state of alert in the weeks before the attack, but chose the weekend of the attack to let his men stand down, rest and carry out maintenance on their equipment. The 60 mobile 3-in (76.2-mm) anti-aircraft guns were parked away from their firing positions and had no ammunition. The stand-down also explained the low availability of army aircraft on the morning of the attack: of the 231 army aircraft on Oahu, some 88 were under repair or maintenance at the time of the attack. Had the Japanese attacked the previous weekend, they would have encountered a garrison prepared to rapidly man its guns and sortie its aircraft.

Another unfortunate decision was to operate the Aircraft Information Center only in training mode until after war broke out, because Short and Kimmel felt they could not spare the officers to run it until that time. This deprived Oahu of its best chance of a timely warning of the Japanese strike.

In greater detail, at this time Kimmel’s Pacific Fleet in Pearl Harbor was centred on Pye’s Battle Force (Task Force 1) comprising Rear Admiral Walter S. Anderson’s Battleships, Battle Force (Rear Admiral Isaac C. Kidd’s Battleship Division 1 with Arizona, Nevada and Oklahoma, Rear Admiral David W. Bagley’s Battleship Division 2 with Pennsylvania, Tennessee and California, and Anderson’s own Battleship Division 4 with Maryland, West Virginia and Colorado, the last currently in Puget Sound Navy Yard undergoing overhaul; Rear Admiral H. Fairfax Leary’s Cruisers, Battle Force comprising part of Cruiser Division 6 with the heavy cruisers New Orleans and San Francisco, and Leary’s own Cruiser Division 9 with the light cruisers Helena, Honolulu, Phoenix and St Louis; and Rear Admiral Milo F. Draemel’s Destroyers, Battle Force with Captain Robert A. Theobald’s Destroyer Flotilla 1 (light cruiser Raleigh, Destroyer Squadron 1 with Phelps, Destroyer Division 1 with Dewey, Hull, Macdonough and Worden, Destroyer Division 2 with Farragut, Dale, Monaghan and Aylwin, Destroyer Squadron 3 with Selfridge, Destroyer Division 5 with Reid, Conyngham, Cassin and Downes, Destroyer Division 6 with Cummings, Case, Shaw and Tucker), and Draemel’s own Destroyer Flotilla 2 (light cruiser Detroit and destroyers Bagley, Blue, Helm, Mugford, Ralph Talbot, Henley, Patterson and Jarvis).

Other destroyers at Pearl Harbor were Allen, Schley, Chew and Ward, the last patrolling the channel entrance to Pearl Harbor.

Also part of the Pacific Fleet were the submarines Narwhal, Dolphin, Cachalot and Tautog, minelayer Oglala, minesweepers Turkey, Bobolink, Rail, Tern, Grebe and Vireo, coastal minesweepers Cockatoo, Crossbill, Condor and Reedbird, destroyer minelayers Gamble, Ramsay, Montgomery, Breese, Tracy, Preble, Sicard and Pruitt, destroyer minesweepers Zane, Wasmuth, Trever and Perry, patrol gunboat Sacramento, destroyer tenders Dobbin and Whitney, seaplane tenders Curtiss and Tangier, small seaplane tenders Avocet and Swan, converted destroyer seaplane tenders Hulbert and Thornton, ammunition ship Pyro, oilers Ramapo and Neosho, repair ships Medusa, Vestal and Rigel, submarine tender Pelias, submarine rescue ship Widgeon, hospital ship Solace, cargo ship Vega, stores issue ships Castor and Antares, and ocean tugs Ontario, Sunnadin, Keosanqua which was just entering Pearl Harbor, and Navajo which was off the entrance to Pearl Harbor.

In the period well before the Japanese attack, the US civilian and military intelligence forces had, between them, secured good information suggesting the high probability of additional Japanese aggression throughout the summer and autumn of 1941, but none of this specifically indicated an attack on Pearl Harbor. During November, both the US Navy and US Army in the Hawaiian islands group were explicitly warned that war with Japan was expected in the very near future and, on the day of the attack, General George C. Marshall, the US Army’s chief-of-staff, sent an imminent war warning message to the senior commanders at Pearl Harbor. In Hawaii, there were several indications of the impending attack, but none resulted in, or even offered enough time to cause, a heightened state of readiness. Had any of this produced an active alert status, the attack might have been resisted more effectively, and perhaps caused less damage. In the event, the attack fell on a Pearl Harbor which was wholly unprepared.

US signals intelligence, through the Army Signal Intelligence Service and the Office of Naval Intelligence’s OP-20-G unit, had intercepted Japanese diplomatic traffic and had broken many Japanese ciphers, but the distribution of this intelligence was poor and did not include material from Japanese military traffic as this was currently not available. As frequently occurs in such cases, the information was partial, seemingly contradictory, or insufficiently distributed (as in the case of the ‘weather code phrase’ or ‘winds’ system used by the Japanese to indicate specific eventualities). Even so, warnings were sent to all US commands in the Pacific during November 1940.

Despite the growing information suggesting the imminent nature of a new phase of Japanese aggression, still there was little information specific to Pearl Harbor. US commanders were warned that tests had shown that shallow-water torpedo air launches were possible, but no senior officer fully appreciated the danger posed by this and other new developments. Confident in Pearl Harbor’s natural defences against torpedo attack (i.e. the harbour’s shallow water), the US Navy failed to add torpedo nets or baffles, which they judged cumbersome and a low priority. As a result of a claimed shortage of long-range aircraft, distant reconnaissance patrols were not being flown.

At the time of the attack, the US Army was concentrating on the training task rather than maintaining a status of high alert. As noted above, most of its mobile anti-aircraft guns were stowed with their ammunition kept locked in separate armouries and, in order not to upset property owners, the guns were not dispersed onto private property.

Part of the Japanese scheme for the attack included the ending of negotiations with the USA at a time 30 minutes before the attack. Diplomats of the Japanese embassy in Washington, including the ambassador Admiral Kichisaburo Nomura and special envoy Saburo Kurusu, had been conducting extended talks with the Department of State regarding the US reactions to the Japanese move into Indo-China during the summer. In the days before the attack, the foreign office in Tokyo sent a long multi-part message (encrypted with the ‘Purple’ cryptographic machine) to the Japanese embassy in Washington, DC, together with instructions for this message to be decrypted and delivered to Hull at 13.00 Washington time, just 30 minutes before the attack was scheduled to begin. The last part of the message arrived not long before the attack, but because of decryption and typing delays, embassy personnel could not adhere to Tokyo’s schedule. This last part broke off negotiations with the assertion that ‘Obviously it is the intention of the US government to conspire with [the UK] and other countries to obstruct Japan’s efforts toward the establishment of peace through the creation of a new order in East Asia…Thus, the earnest hope of the Japanese government to adjust Japanese-American relations and to preserve and promote the peace of the Pacific through co-operation with the US government has finally been lost’, and was in fact delivered to Hull well after the intended time.

Japanese records admitted into evidence during the following Congressional hearings on the attack established that the Japanese had not even written a declaration of war until after they heard of the successful attack on Pearl Harbor. That two-line declaration of war was finally delivered to Ambassador Joseph C. Grew in Tokyo only some 10 hours after the attack had been completed. Grew was permitted to transmit it to the USA, where the declaration was received late in the afternoon of Monday.

The USA had decrypted the last part of the final message well before the Japanese embassy managed to achieve the same task, and long before the embassy had completed a finished typed copy of the decrypt. It was the decryption of the last part which had prompted Marshall to send his warning to Hawaii that morning. It was actually delivered to Short several hours after the attack had ended, the delay being occasioned by the fact that Marshall was out riding, and that the US Army decided not to use US Navy facilities to transmit the warning. After its own long-distance communications system encountered difficulties, the US Army then elected to use commercial cable.

The first shots fired and the first casualties in the attack on Pearl Harbor actually occurred when the destroyer Ward attacked and sank a midget submarine at 06.37 local time. The five midget submarines in the Japanese plan were intended to torpedo any US ships seeking to leave Pearl Harbor after the air attack began. None of the midget submarines made it back safely, and the wrecks of only four of the five have since been found. Of the 10 men crewing the five submarines, nine died, and the only survivor was captured to become the first prisoner of war taken by the Americans in World War II. Photographic analysis by the US Naval Institute in recent years indicates that one of the midget submarines did manage to enter Pearl Harbor and successfully fire a torpedo into the battleship West Virginia. The final disposition of this submarine is unknown.

The encounter between Ward and the midget submarine was not the only warning the Americans missed. The Japanese launched two scout floatplanes ahead of the first attack wave to reconnoitre Pearl Harbor and Lahaina Roads, the Pacific Fleet’s alternative anchorage. This was a pointless effort as the attack force had little time to take advantage of their report and, moreover, their presence might have given away the Japanese element of surprise.

The US Army’s radar equipments were being operated from 04.00 to 07.00, but as noted above the associated fighter direction centre was not yet fully established and its personnel were still being trained. When the radar operators at Opana Point, on the northern tip of Oahu, picked up a large formation of aircraft approaching from the north just after 07.00, the only officer still at the fighter control centre dismissed the sighting as a flight of B-17 bombers due from the mainland. For security reasons, he did not explain this to the radar operators, and thus denied them the opportunity to report that this formation consisted of at least 50 aircraft, far more than the number of expected B-17 bombers.

Several US aircraft were shot down as the Japanese air attack approached: of these at least one radioed a somewhat incoherent warning. Other warnings were still being processed or awaiting confirmation when the attack began.

Commander Mitsuo Fuchida, leading the first attack wave, had prepared two attack options between which he would chose depending on whether or not surprise was achieved. In the event of complete surprise, he would fire a single smoke signal from the cockpit of his aeroplane, indicating that the torpedo bombers should attack first, before the Americans were alerted. The dive-bombers and fighters would then carry out their airfield attacks. If surprise was lost, Fuchida would fire two smoke signals, indicating that the fighters and dive-bombers should attack first and attempt to distract and suppress the US anti-aircraft defences before the torpedo bombers made their runs in vulnerable low and straight flight. Fuchida accordingly fired a single smoke signal for the surprise option, but his signal was missed by the torpedo aircraft. When Fuchida saw that they were not deploying for the attack, he felt compelled to repeat his smoke signal. Both smoke signals were seen by the fighters and dive-bombers, whose pilots assumed surprise had been lost and immediately deployed for attack. The result was chaos. The first aircraft to reach their release point were dive-bombers assigned to attack Ford Island in the middle of Pearl Harbor, and the explosion of their bombs gave the Americans perhaps four or five minutes warning before the torpedo bombers reached 'Battleship Row', and this may have prevented an even greater US catastrophe.

The attack on Pearl Harbor proper began at 07.53 on Sunday 7 December, initial guidance being found in the music broadcast of a local radio in Pearl Harbor. The Japanese aircraft attacked in two waves, in which a total of 353 aircraft reached Oahu. Fuchida’s first wave of 184 machines comprised Fuchida’s own 1st Group (1st to 4th Attack Units with 49 B5N aircraft each carrying one 1,764-lb/800-kg armour-piercing bomb for high-altitude release, and 1st to 4th Torpedo Attack Units with 40 B5N aircraft each carrying one Type 91 torpedo), Lieutenant Commander Takahashi’s 2nd Group (15th and 16th Attack Units with 51 D3A aircraft each carrying one 551-lb/250-kg HE bomb), and Lieutenant Commander Itaya’s 3rd Group (1st to 6th Fighter Combat Units with 44 A6M fighters for air control and strafing).

The first attack wave was divided into six subformations: of the 1st Group, all of the B5N aircraft were to attack the ships of the Pacific Fleet; of the 2nd Group, the 15th Attack Unit was to attack the hangars and aircraft on Ford Island and the 16th Attack Unit the hangars and aircraft on Wheeler Field; and of the 3rd Group, the 1st and 2nd Fighter Combat Units were to attack Ford Island and Hickam Field, the 3rd and 4th Fighter Combat Units were to attack Wheeler Field and Barbers Point, and the 5th and 6th Fighter Combat Units were to attack Kaneohe. It was planned that each of the attack groups would start with the bombers to secure the maximum element of surprise, and end with the fighters to strafe and deter any pursuit. Within this concept it was intended that the vulnerable torpedo bombers and level bombers should lead the way, hoping to exploit the first moments of surprise by attacking the carriers (believed to be in harbour) and battleships, while the dive-bombers and fighters attacked the US air bases across Oahu, starting with Hickam Field, the largest, and Wheeler Field, the principal fighter base, and also attacking Ewa Field.

As the surviving Japanese aircraft of the first wave pulled out at 08.35, the battleship West Virginia was sinking, the battleship Arizona had settled on the bottom, the battleship Oklahoma had capsized, the battleship Tennessee was on fire, and the damaged battleship Nevada was making for the harbour mouth.

Already a second attack force was approaching the harbour, this formation under Lieutenant Commander Shigekazu Shimazaki comprising 36 A6M, 54 B5N and 81 D3A machines. These were organised as Shimazaki’s own 1st Group (5th and 6th Attack Units with 54 B5N aircraft each carrying one 551-lb/250-kg anti-ground and two 121-lb/55-kg general-purpose bombs), Lieutenant Commander Egusa’s 2nd Group (11th to 14th Attack Units with 78 D3A aircraft each carrying one 551-lb/250-kg bomb) and Lieutenant Commander Shindo’s 3rd Group (1st to 4th Fighter Combat Units with 36 A6M fighters for air control and strafing). The second wave was divided into a 1st Group whose 5th Attack Unit was to attack aircraft and hangars on Kaneohe, Ford Island and Barbers Point and a 6th Attack Unit that was to attack hangars and aircraft on Hickam Field, a 2nd Group whose 11th to 14th Attack Units were to attack targets of opportunity in and around Pearl Harbor, and a 3rd Group whose 1st and 2nd Fighter Combat Units were to attack Ford island and Hickam Field, and 3rd and 4th Fighter Combat Units to attack Wheeler Field and Kaneohe.

The separate sections arrived at their attack points almost simultaneously and from several directions. The overall pattern of the second wave’s attack was similar to that of the first wave when it started at 09.15, and Nevada was forced to beach herself while the dry-docked battleship Pennsylvania was severely damaged. The second-wave aircraft also attacked Bellows Field, Naval Air Station Kaneohe and NAS Ford island, the last a US Marine and US Navy air base in the middle of Pearl Harbor.

The only opposition came from some Curtiss P-36 and Curtiss P-40 fighters, which flew 25 sorties, and from naval anti-aircraft fire.

About 90 minutes after its start, the attack was over. Some 2,403 Americans were dead, this figure including 68 civilians, of whom many were killed by American anti-aircraft shells falling back onto Honolulu, and another 1,178 had been wounded. The Japanese had sunk three battleships, caused one battleship to capsize, and severely damaged four battleships, three light cruisers and three destroyers. Nearly half of the US dead, some 1,102 men, happened in the explosion and sinking of the battleship Arizona, which was destroyed when a converted 16-in (406-mm) shell, dropped at high altitude by a level bomber, smashed through her two armoured decks and detonated the forward main magazine.

The battleship Nevada attempted to leave the harbour and get out to sea, but was ordered to beach herself at Hospital Point to avoid blocking the harbour entrance should she sink. Already damaged by a torpedo and on fire forward, Nevada was targeted by many Japanese bombers as she steamed away, and sustained more hits from 250-lb (113-kg) bombs as she beached.

The battleship California was hit by two bombs and two torpedoes, and the crew might have been able to keep her afloat has they not been ordered to abandon ship just as they were raising power for the pumps. Burning oil from Arizona and West Virginia was drifting down on her, and over a period of three or four days she sank slowly to end with her quarterdeck under some 12 ft (3.6 m) of water.

The disarmed battleship Utah, used as a target ship, was holed twice by torpedoes and capsized.

The battleship West Virginia was hit by no less than seven torpedoes, the seventh tearing off the ship’s rudder, and sank at her berth.

The battleship Oklahoma was hit by four torpedoes, the last two impacting above her belt of side armour, and capsized with a small part of her hull remaining above the water.

The battleship Maryland was hit by two converted 16-in (406-mm) shells, but neither of these caused serious damage.

The battleship Tennessee was badly damaged aft as a result of a fire, and was otherwise moderately damaged.

The battleship Pennsylvania was in dry dock at the time of the attack, and suffered major damage.

Although they concentrated their efforts on the battleships, the Japanese did not ignore other targets. The light cruiser Helena was torpedoed, and the concussion from the blast capsized the neighbouring minelayer Oglala. The destroyers Cassin and Downes were in dry dock, ahead of Pennsylvania: bombs penetrated their fuel tanks, the leaking fuel caught fire, and flooding of the dry dock only made the oil rise, which burned out the ships. The light cruiser Raleigh was hit by a torpedo and holed. The light cruiser Honolulu was damaged but returned to service. The destroyer Shaw had her bow blown off even as she was in dry dock, and was severely damaged. The repair vessel Vestal was heavily damaged and beached, and the seaplane tender Curtiss was damaged.

Of about 390 US aircraft in the Hawaiian islands group, 188 were destroyed and 155 damaged. Attacks on barracks killed some aircrew, and ‘friendly fire’ brought down several US aircraft. American casualties were 2,402 military personnel killed and 1,247 wounded, together with 57 civilians killed and 35 wounded.

Some 55 Japanese airmen and nine submariners were killed in action. Of Japan’s 414 aircraft, 350 took part in the attack and 29 were lost (nine in the first attack wave and 20 in the second wave in the form of nine A6M, 15 D3A and five B5N machines) and another 74 were damaged by ground fire. Another 20 Japanese aircraft which regained their carriers were deemed irreparable.

Back on the Japanese carriers, some of senior officers and flight leaders urged Nagumo to launch a third attack to destroy Pearl Harbor’s oil tank farms, machine shops and dry docks. The US Navy had considered the vulnerability of the fuel oil storage tanks before the war and secretly started construction of the bomb-resistant Red Hill fuel tanks before the Japanese attack. Destruction of these facilities would have greatly increased the US Navy’s difficulties as the nearest immediately usable fleet facilities were those several thousand miles to the east of the Hawaiian islands group on the west coast of the continental USA. Nagumo decided not to launch a third attack, however, but instead to pull back his force for eight reasons. First, the US anti-aircraft performance during the second strike was much improved over that during the first, and two-thirds of the Japanese losses happened during the second wave as a result, at least in part, to the fact that the US forces were now on the alert and any third strike could have suffered still worse losses. Second, the first two attacks had used all the available aircraft previously prepared for action, so a third attack would have taken some time to prepare, allowing the US forces the time, perhaps, to find and attack the ships of the Pearl Harbor Strike Force, especially as the location of the Pacific Fleet’s three aircraft carriers was still not known. Third, the Japanese pilots had not trained for any attack on Pearl Harbor’s shore facilities, and the organisation of such an attack would have taken still more time, though several of the strike leaders nonetheless urged that the risk inherent in a third attack were worthwhile. Fourth, the fuel situation did not permit an extended loiter to the north of Pearl Harbor, for the Japanese were operating at the limit of their logistical ability to support the Pearl Harbor attack, and to remain in those waters for much longer would have risked running unacceptably low on fuel. Fifth, the timing of a third strike would have been such that aircraft would probably have returned to their carriers after dark: night operations to and from aircraft carriers were in their infancy in 1941, and there were no reliable techniques for such operations. Sixth, the second attack had essentially completed the entire mission by neutralising the Pacific Fleet. Seventh, there was the simple danger of remaining in one area for too long, and Nagumo felt that the Japanese had been very fortunate to escape detection during their voyage from the Inland Sea to Hawaii: the longer they remained off Hawaii, the more danger they were in from US submarines and the absent American carriers. Eighth, the carriers of the 1st Air Fleet were now needed to provide either close or distant support for the main Japanese offensives toward the ‘Southern Resources Area’ (‘M’ against the Philippine islands group, ‘B’ [ii], ‘H’, ‘J’, ‘L’ and ‘T’ against the Netherlands East Indies, ‘E’ against Malaya and ‘B’ [iii] against Burma), whose seizure was to provide vitally important oil and other supplies.

The Japanese government had been reluctant to allow the attack at all as it took air cover from the southern thrust, and Nagumo was under strict orders not to risk his command any more than necessary. As the war games during the planning of the attack had predicted that from two to four carriers might be lost in the attack, Nagumo must have been very happy to have suffered no ship losses and did not want to push his luck.

Just 90 minutes before the start of the attack on Pearl Harbor (but the next day, 8 December, on the other side of the international date line), Japanese troops invaded the British possession of Malaya. This was followed by an early morning attack on the New Territories of Hong Kong and within hours or days by attacks on the Philippines islands group, Wake island and Thailand, and by the sinking of a British force comprising the battleship Prince of Wales and battle-cruiser Repulse.

On 8 December the US Congress declared war on Japan, only one strongly pacifist congresswoman casting a dissenting vote. The USA was totally outraged by the attack and by the late delivery of the note breaking off relations, actions which it considered treacherous. Roosevelt signed the declaration of war on the same day, and called the previous day ‘a date which will live in infamy’ in an address to a joint session of the Congress. Continuing to intensify its military mobilisation, the US government ordered the start of conversion to a full war economy.

The Pearl Harbor attack immediately galvanised a divided nation into action as little else could have done. Overnight, it united Americans against Japan, and it probably rendered feasible the unconditional surrender position taken by the Allied powers. Germany declared war on the USA on 11 December. Adolf Hitler was under no obligation to do so under the terms of the Tripartite Pact with Italy and Japan, but nonetheless did so, perhaps as a result of overconfidence. This declaration doubly outraged the American public and allowed the USA to enter the European theatre of war, and to step up its support of the UK, actions which delayed for some time a full US response to the setback in the Pacific.

In terms of its objectives, the Japanese attack on Pearl Harbor had been a tactical success which eclipsed the expectations of its planners. As a result of its grievous losses at Pearl Harbor and the subsequent Japanese invasion of the Philippine islands group, the US Navy was unable to play any significant role in the Pacific War for the following six months. With the Pacific Fleet essentially out of the picture, Japan moved forward to the seizure of South-East Asia and the South-West Pacific, and to extend its reach far into the Indian Ocean.

Despite the perception of this battle as a devastating blow to the USA, however, only three ships were permanently lost to the US Navy. These were the current battleships Arizona and Oklahoma, and the old battleship Utah in service only as a target ship. Nevertheless, much usable material was salvaged from them, including Arizona's two after main turrets. Four ships sunk during the attack were later raised and returned to duty, these including the battleships California, West Virginia and Nevada. California and West Virginia had effective torpedo-defence arrangements which held up remarkably well, despite the weight of fire they had been compelled to endure, allowing most of their crews to be saved. Many of the surviving battleships were extensively refitted and modernised, allowing them to cope better with Japanese threats.

The destroyers Cassin and Downes were constructive total losses, but their machinery was salvaged and fitted into new hulls, which also received their names, while Shaw was raised and returned to service.

Of the 22 Japanese surface warships involved in the attack (this total including the destroyers Ushio and Sazanami in the related attack on Midway island by the Midway Destruction Unit), only one survived the war.

In the long term, the attack on Pearl Harbor can be seen only as an unmitigated strategic blunder for Japan. Yamamoto, who devised the Pearl Harbor attack, had predicted that even a successful attack on the US fleet would not and could not win a war with the USA as American productive capacity was too great. One of the main Japanese objectives was to destroy the three US aircraft carriers stationed in the Pacific as the core of Halsey’s Aircraft, Battle Force, Pacific Fleet, but neither these nor the Pacific Fleet’s heavy cruisers were present at Pearl Harbor: Enterprise of Halsey’s own Carrier Division 2 was returning from a cruise, Lexington of Rear Admiral Aubrey W. Fitch’s Carrier Division 1 had sortied a few days earlier and Saratoga also of Carrier Division 1 was in San Diego following a refit at Puget Sound Navy Yard.

Putting most of the US battleships out of commission was widely regarded at the time as a huge success for the Japanese. However, the elimination of many of its battleships left the US Navy with no alternative but to put its faith in aircraft carriers, escorted by cruisers and destroyers, as well as submarines, which were the weapons now left to it, and these now became the ‘capital ships’ with which the US Navy halted and then reversed the Japanese advance. A particular flaw of Japanese strategic naval thinking was that the ultimate Pacific battle would be fought between the battleships of the two contestants, and as a result Yamamoto hoarded his battleships for a decisive battle which would not, in fact, ever take place.

Ultimately, however, targets that the Japanese had decided not to attack, namely the Submarine Base and the old Headquarters Building, were more important than any of them. It was submarines which ultimately brought Japan’s economy to a standstill and crippled its transportation of oil, immobilising the Japanese navy’s heavy warships. And in the basement of the old Headquarters Building was Hypo, the US Navy’s all-important cryptanalytical unit.