This was the Japanese strategic plan for the defence of the home islands against any Allied invasion, in fact being planned by the Americans as ‘Downfall’ (15 April/15 August 1945).
Tokyo was clearly the primary Allied objective, and to protect their capital the Japanese allocated Field Marshal Hagime Sugiyama’s 1st General Army, headquartered in Tokyo, to control the three army groups in the central portion of Honshu island: from south-west to north-east these were Lieutenant General Tasuke Okada’s 13th Area Army headquartered at Nagoya and controlling six infantry divisions, Lieutenant General Keisuke Fujie’s (from 9 March General Shizuichi Tanaka’s) 12th Area Army headquartered in Tokyo and controlling two armoured and 18 infantry divisions, and Lieutenant General Teiichi Yoshimoto’s 11th Area Army headquartered in Sendai and controlling six infantry divisions. The reserve for this area was Lieutenant General Toshimichi Uemura’s 36th Army, and General Masakazu Kawabe’s Air General Army controlled such air assets as were left in the theatre.
The defence of the rest of the home islands rested with Field Marshal Shunroku Hata’s 2nd General Army, headquartered in Hiroshima, and this controlled General Kiichiro Higuchi’s 5th Area Army headquartered in Sapporo and defending the island of Hokkaido in the north with five infantry divisions, General Masakazu Kawabe’s (from 7 April Lieutenant General Eitaro Uchiyama’s) 15th Area Army headquartered in Osaka and defending the eastern tip of Honshu and the island of Shikoku just to its south with eight infantry divisions, and Lieutenant General Isamu Yokoyama’s 16th Area Army headquartered in Fukuoka and defending the south-eastern island of Kyushu with two armoured and 14 infantry divisions.
The Japanese had appreciated the likely disposition of the Allied invasions, for their four available armoured divisions were allocated to two primary threat areas: Kyushu where ‘Olympic’ was to happen, and the Tokyo area where ‘Coronet’ (ii) was to take place. In all, the forces allocated to the defence of central Honshu and of Kyushu were about half of those available, and amounted to the major formations listed above plus 25 independent mixed brigades, three guards brigades and seven tank brigades. Though Japan would have been much depleted in weapons by the time of the US invasions, there was little doubt in the minds of the US and Japanese planners alike that the determination and numbers of the Japanese defenders would cost the Americans well over 1 million casualties before Japan could be beaten into total defeat.
Given the fact that the considerably greater part of Japan’s troop strength was located overseas and industrial production in the home islands was suffering severely under constant US bombing attacks, the defence of the Japanese home islands presented an enormous challenge to the Imperial General Headquarters. On 8 April 1945, this issued orders, effective from 15 April, to activate Sugiyama’s 1st General Army and Hata’s 2nd General Army to control the ground defence of the home islands, with air support provided by Kawabe’s Air General Army, which was to co-ordinate the air defence of the home islands by the provision of a single headquarters through which co-operation with the ground forces and the navy could be expedited for the rapid and effective defence of the home islands. Simultaneously with the activation of these three high-level formations, the Imperial General Headquarters issued orders for the implementation of 'Ketsu'. This latter was wholly defensive, and divided the home islands into seven zones from which to fight the final decisive battles of the Japanese empire.
The strategy for 'Ketsu' was outlined in an army directive of 8 April. This stated that the army would seek to crush the US invasion forces while they were still at sea. This, the Japanese hoped, they would achieve by the delivery of a decisive blow against the US naval forces by the initial destruction of as many of the US carriers as possible by means of a massive deployment of the kamikaze aircraft of the army and navy air arms' 'special attack forces'. When the US amphibious forces came within range of the air bases of the home islands, the entire Japanese air combat strength would be employed in continual night and day assaults against these ships, emphasis being placed on the disruption of the US assault landing plans. The principal targets in this phase of the undertaking were to be the vessels carrying the troops and their weapons and other equipment. Any US forces which did succeed in landing would then be swiftly attacked by the army with a view to securing a decisive victory by destroying the US landing forces on their assault beaches before they could advance inland and establish secure lodgements.
'Ketsu' was therefore conceived as a joint defence effort of total commitment to be fought by the entire strengths of the army and navy, and the many orders and directives which now emanated from the Imperial General Headquarters stressed the vital importance of full and unstinting inter-service co-operation, which was something which the Japanese forces had seldom managed up to this time.
The basic plan for the operation demanded that the navy defend the coast by attacking the invasion fleets with its combined surface, submarine and air forces, a task in which the Air General Army would co-operate closely in locating the US transports vessels and destroying them at sea. Should the invasion force succeed in making a landing, the relevant area army would assume command of all naval ground forces in its area and exercise operational control of the air forces in support of ground operations. An integral part of the 'Ketsu' planning included the reinforcement of sectors under attack by units transferred from other areas. As US air attacks had already seriously disrupted the entire land and coastal transportation systems of the home islands, time schedules were planned on the basis of all troop movements being made on foot. If the battle on the assault beaches showed no prospect of an outcome favourable to the Japanese, the battle would inevitably shift to inland warfare, an eventuality for which special planning was made. Guard units and civilian defence corps personnel, with elements of the field forces acting as their nuclei, were to be used as interior resistance troops with the task of wearing down the US forces by means of guerrilla warfare, espionage, deception, dislocation of supply areas, and interdiction of supply routes and services as the US invasion forces started to advance inland.
The operational preparations for 'Ketsu' were undertaken in three phases. The first, in which defensive preparations and troop unit organisation were completed, continued throughout July 1945 and were completed. The second and third phases were not completed as their implementation was overtaken by the end of the war, but the second phase, in which training was undertaken and all defences improved, began in August and was intended to continue throughout September. The third phase, which troop training and deployment, as well as the construction of all defence positions, was to have been completed during October. Thus, if implemented, the 'Olympic' landing on Kyushu island, scheduled for 1 November, would have started just as the Japanese defence plans had been completed.
In 'Olympic', Lieutenant General Walter C. Krueger’s US 6th Army would have landed against elements of the 2nd General Army, whose task it was to hold the western portion of Honshu and the islands of Shikoku and Kyushu from its headquarters established at Hiroshima on 18 April. The 2nd General Army controlled General Kiichiro Higuchi’s 5th Area Army headquartered in Sapporo and defending the island of Hokkaido in the north with five infantry divisions, General Masakazu Kawabe’s (from 7 April Lieutenant General Eitaro Uchiyama’s) 15th Area Army headquartered in Osaka and defending the eastern tip of Honshu and the island of Shikoku just to its south with eight infantry divisions, and Lieutenant General Isamu Yokoyama’s 16th Area Army headquartered in Fukuoka and defending the eastern island of Kyushu with two armoured and 14 infantry divisions.
Each of the seven defensive zones established for 'Ketsu' had its own individual defensive plan. The defence of Kyushu was this 'Ketsu No. 6'. While Kyushu had previously fallen within the Western Military District, in 'Ketsu No. 6' the defence of Kyushu became the responsibility of the 16th Area Army under the 2nd General Army.
The 2nd General Army developed its defence plan on the estimation that the US forces would enlarge their foothold on Okinawa, establish air bases on that island and, as soon as possible, begin their drive at the Japanese home islands via southern Kyushu. It was believed that the first US objective would be to secure operational bases for its naval and air forces, and thus the Japanese correctly reckoned that the US initial objective would be to secure the Kagoshima Wan to provide the anchorage and port facilities necessary for the US build-up. The Japanese reckoned that the earliest possible time at which a US invasion attempt might be made was the first half of July, when it was estimated that a strength of 10 divisions could have been assembled. By July, Japanese officers were assessing that the invasion would occur during October or November 1945 after the end of the summer typhoon season.
The concept behind 'Ketsu' was the infliction of huge numbers of casualties on the US invasion forces, thereby undermining the US people’s will to continue the fight for Japan’s unconditional surrender. This intent was clear in a number of sources, but is nicely encapsulated in the statement of a staff officer of the Imperial General Headquarters in July 1945: 'We will prepare 10,000 aircraft to meet the landing of the enemy. We will mobilise every aeroplane possible, both training and "special attack" aircraft. We will smash one third of the enemy’s war potential with this air force at sea. Another third will also be smashed at sea by our warships, human torpedoes and other special weapons. Furthermore, when the enemy actually lands, if we are ready to sacrifice 1 million men we will be able to inflict an equal number of casualties upon them. If the enemy loses 1 million men, then US public opinion will become inclined toward peace, and Japan will be able to gain peace with comparatively advantageous conditions.'
This statement makes it fully clear that in the summer of 1945 Japanese strategists had identified the will of the US people as the US strategic centre of gravity and therefore critically vulnerability to severe US casualty figures.
The completion of defensive preparations in Kyushu was a matter of the greatest urgency as the initial US attack would almost inevitably be directed at that island. Kyushu is the the south-westerly of the main Japanese home islands, and is about 200 miles (320 km) long from north to south and 125 miles (200 km) wide from east to west, and in 1944 had a population in the order of 10.041 million people. most of them concentrated in the industrial north and north-west centred on the cities of Sasebo, Nagasaki and Fukuoka. The southern part of the island, whose main population centres were Sendai in the east and Miyazaki in the west, is separated from the north by rugged mountains that restrict communications to the coasts. The island’s defence was the most problematical of those of all the districts, as Kyushu had the greatest length of vulnerable sea coast to be defended. Since it was generally conceded that the US forces would make their initial landings in Kyushu, the 16th Area Army had been given priority in the receipt of supplies and in the build-up of troop strength. The construction of fortifications had also been emphasised and, in overall terms, defensive preparations were further advanced in Kyushu than in any other area of Japan.
While 'Ketsu No. 6' was the overall scheme for the defence of Kyushu, within this the 16th Area Army prepared its own detailed defence plan. This 'Mutsu No. 1' divided Kyushu into three sectors which were, in turn, broken down into seven sub-divisions. The 16th Area Army estimated that the main US landing effort would be directed against the south-eastern part of the coast near Miyazaki, with secondary assaults anticipated in the Ariake Wan and along the south-western part of the coast at Fugiachi Hama on the Satsuma peninsula. 'Mutsu No. 1' was given priority over other operations, and the Japanese were indeed extremely accurate in their estimation of the locations at which the Americans intended to make their landings.
Deployed throughout Kyushu and on adjacent islands, the 16th Area Army had three armies and two special forces with a total of 15 divisions, seven independent mixed brigades, three independent tank brigades and two fortress units. The defence plan called for each of the armies to hold one division in reserve. In the event of an invasion, the 16th Area Army would concentrate a force composed principally of its subordinate armies' reserve divisions and the three tank brigades. This force would then be used as an assault grouping which would be rushed to the area of the main US effort with the object of destroying the US forces as soon as possible after their initial landings. The defensive plan called for a major counter-offensive to be delivered within two weeks of the initial US landings. A Japanese officer stated that the object of the defence was 'to frustrate the enemy’s landing plans with a counterattack like an electric shock, and at the proper moment to annihilate the enemy by close-range fire, by throwing hand grenades, and by hand-to-hand combat.' Groups assigned to coastal defence were to contain the Americans as the reserve formations were being concentrated for the decisive battle or, in some cases, to hold out for considerable periods until a decisive battle was won in some other area and permit the release of strength for a counterattack in the sector being held.
Possessing nothing with which to offset US air superiority, the Japanese planned to exploit everything they could to confuse the battle lines so as to prevent the use of naval gunfire and air power in support of the US ground forces. The advances of the mobile reserves would be accomplished under cover of darkness for protection from US air power.
The defence positions in Kyushu were built in accordance with the precepts laid down in The Three Basic Principles on How to Fight Americans, which had been developed as a result of lessons learned in the campaigns of the South Pacific theatre. These principles were firstly that positions should be constructed beyond the effective range of US naval bombardment, secondly that cave-type positions should be constructed for protection against air raids and naval bombardment, and thirdly that inaccessible high ground should be selected as protection against flame-throwing tanks.
The production, movement and distribution of weapons, ammunition and supplies was one of the most important aspects of the defence preparations on Kyushu, and the preparations included the creation of munition dumps in caves and other underground shelters to protect them from air raids and naval bombardment. The original Japanese plan called for each division to be supplied with one campaign unit of fire, and by July 1945 this quantity was actually in the possession of the area armies. One campaign unit of fire was sufficient ammunition for one campaign, and was generally understood to be a quantity sufficient for three months of fighting, and comprised 1,000 rounds per field piece, 25,000 rounds per machine gun, and 240 rounds per rifle. However, by August 1945 the greatly increased number of troops on Kyushu made it necessary to reduce ammunition stocks to a half unit of fire for each unit, sufficient for about six weeks of combat. This reduction in turn required the adjustment of supply plans for the high-priority areas and to plan for the rapid transfer of ammunition from one area to another when the invasion was actually launched and the place and direction of attack had been determined. The Japanese were preparing and may in fact have been able to bring their ammunition supplies back up to the three-month level had the war continued and 'Olympic' been undertaken in November.
Air operations against the US landings on Kyushu were to be the responsibility of Vice Admiral Matome Ugaki’s (from 17 August Vice Admiral Ryunosuke Kusaka’s) 5th Air Fleet and the 6th Air Army under the overall control of the Air General Army. The two air formations had the use of airfields throughout Kyushu, Shikoku and the Chugoku region at the western end of Honshu. Airfields in southern part of Kyushu, which were being attacked almost daily, had been abandoned as bases and were to be employed only for the staging of kamikaze missions to neutralise as many transports as possible as the US invasion fleet approached the shores of Japan. If landings were made, the air forces would conduct operations to sever the US supply lines and thereby ease the task of the Japanese ground forces. Aircraft were to be committed in waves 300 to 400 machines, at the rate of one wave per hour, against the invasion fleet. Sufficient fuel had been stored for this, but only about 8,000 pilots were available. Although the pilots were poorly trained and no match against experienced US pilots, they were deemed capable enough to carry out kamikaze attacks on ships.
At the end of the war, Japan still possessed some 12,725 aircraft, 5,651 and 7,074 in army and navy service respectively. While many of these could be in no way regarded as warplanes, almost all had been converted for the kamikaze role. The Japanese were planning to train enough pilots to man all of the aircraft which were capable of flight.
Naval operations against the invasion fleet were to be undertaken in two phases. The first was to comprise attacks to reduce the US fleet as it approached the home islands. Within this, the remaining 38 Japanese fleet submarines were to try to sink as many transport vessels as possible, and were also to serve as launch platforms for 'Kiten' manned suicide torpedoes. Although these latter proved none too successful in open-water operations, the Japanese hoped that they would be more effective in the restricted waters around the home islands. 'Koryu' five-man midget submarines were also to be employed with either two torpedoes or an explosive charge for use in a suicide role. The navy planned to have 540 'Koryu' boats in service by the time of the invasion. A more advanced midget submarine, the 'Kairyu' was a two-man craft armed with either two torpedoes or an explosive charge. It was planned that there would be some 740 'Kairyu' in service by the autumn of 1945.
The second phase was to be started as the US invasion fleet reached the landing areas. The 19 surviving Japanese destroyers were to attack the US transport vessels at the invasion beaches. 'Shinyo' kamikaze attack boats, each carrying a 551-lb (250-kg) explosive charge in its bow, were to attack from concealed lairs along the coast with a view to hitting and sinking any craft carrying troops. The Japanese navy and army had an estimated total of 3,300 kamikaze 'special attack' boats between them. Finally, there would be rows of 'Fujuryu' kamikaze frogmen in their diving gear some 33 ft (10 m) below the surface. The outermost row of 'Fukuryu' divers were to release anchored mines or carry mines to craft that passed nearby. Closer to shore, there were to be three rows of divers, arrayed so that they were about 66 ft (20 m) apart. Underwater lairs for the 'Fukuryu' were to be made of reinforced concrete with steel doors, and as many as 18 divers could be stationed in each underwater 'foxhole'. Wearing a diving suit and breathing from oxygen tanks, each 'Fukuryu' diver was equipped with an explosive charge mounted on a stick and fitted with a contact fuse, and was to swim up to landing craft and detonate the charge. The navy had hoped for 4,000 men to be trained and equipped for this suicide force by October.
Land operations against the US landings called for the ground forces to determine as rapidly as possible the area in which the invasion was to be made and to concentrate in this area as many troops as possible before the invasion actually started. If a US preliminary bombardment or early seizure of small islands to the south and south-west of Kyushu indicated an invasion attempt on the southern part of Kyushu, then Lieutenant General Masao Yano’s 57th Division, the 4th Independent Tank Brigade, the 'Chikugo' Force and the 'Higo' Force were to move to the south to the vicinity of Kirishima to counterattack the US beach-head. The main body of infantry was to be arrayed on the first commanding ground inland from the beach, and was to conduct operations designed to destroy the US forces in coastal areas before they could secure firm beach-heads and develop these into a lodgement. Should the US forces advance simultaneously in several locations, the ground forces were to direct their main operation against the primary US force. If the US main force could not be located, then the Japanese would seek a decisive battle in an area where their main force could most easily be directed. In the other operational areas, elements would carry on delaying actions in order to facilitate the operations of the main Japanese force.
Heavy and medium artillery were to cover landing craft approaches, beaches and plain areas surrounding the beaches. Plans for the employment of artillery seemed to combine the beach defence tactics employed on Saipan in the US 'Forager' campaign with some of the fixed defence plans employed on Iwo Jima against the US 'Detachment' campaign. Coast defence and artillery batteries were to withhold their fire until landing craft came in range. However, there was no centralised control or fire-direction of the coast defence and artillery batteries. The Japanese considered the massing of fire as a waste of ammunition, and each artillery position was to remain in place firing independently until destroyed. Artillery and mortar units were to be emplaced generally on the reverse slope of the first ridges inland from the beach and in caves farther inland. The priority for employment of mortars was beach defence.
Commanders were told to be ready to divert the necessary troops and military supplies to other sectors as swiftly as possible at any time. The ground forces were to be concentrated in planned operational areas. Movement of ground forces would be primarily at night and on foot, and the movement of supplies would be by rail or water as the situation permitted. Troop movements were to be executed even under US air attack.
By this stage of the war, the Japanese had gained considerable experience in the manner in which the US forces conducted amphibious assaults in the Pacific. Late in 1944, the Japanese also sent a team of officers to debrief the Germans on their defences in Normandy and how the Allies had assaulted these to gain their 'Overlord' beach-heads and then lodgement in North-West Europe. Drawing on these sources, the Japanese planned their coast defences of Kyushu into three zones.
The first of these comprised the beach positions, which were to be used mainly in beach fighting and for firing against landing craft. They were to be heavily fortified and concealed for protection against naval gunfire bombardment. Coastal fortifications were constructed in cave-type shelters to withstand intense bombings and bombardments, especially from naval gunfire. They were to have the ability to conduct close-range actions and withstand attacks from flame-throwers, explosives, and gas. Their purpose was to defeat any landing attempt.
The second comprised the foreground zone. If the beach positions could not prevent a landing, then the attack was to be delayed in this zone with localised counterattacks and raids. Obstacles, hidden positions, time-fused land mines, and assault tunnels making use of natural terrain features were prepared to slow the attack and to fight within the US lines to limit the effectiveness of naval gunfire and close air support.
The third was the main zone of resistance, which was the area where the main resistance was to be established. Battalions and larger units were to hold key terrain positions independent of each other. These positions were to be organised primarily for anti-tank warfare and their fields of fire were to be short. These installations were constructed as underground fortresses capable of coping with close-range actions in which flame-throwers, explosives and gas would be used. This resistance zone was intended to halt the US advance and allow for the preparation of the major counterattack which was to inflict a decisive defeat on the attack. The Japanese paid very careful attention to the camouflaging of their positions even during construction, and the defensive positions were concealed from air, land and sea observation.
Within all three zones, dummy positions were to be constructed for deception purposes. Cave installations were to be heavily reinforced and capable of withstanding a direct hit by naval gunfire. Pillboxes, assault positions, sniper positions and obstacles were to be organised for close-range combat and also to be mutually supporting. Each position was to store water, ammunition, fuel, anti-tank weapons, food, salt, vitamin pills and medical supplies.
Defensive measures taken farther inland included rear defence zones, which were created in important inland areas as alternate positions for the area army to use in holding out against a major US penetration or in support of a strategic counter-offensive. Blocking positions were also built across lines of communication to check rapid advances by US mechanised forces.
Inland fortifications were also constructed to provide cover and concealment for heavy equipment such as tanks, motor vehicles and artillery, as well as bomb-proof storage of ammunition and fuel. As on many islands throughout the Pacific war, these storage shelters were impervious to US air and naval bombardment.
'Mutso No. 1' covered the defence of southern Kyushu by Lieutenant General Mitsuo Nakasawa’s 40th Army and Lieutenant General Kanji Nishihara’s 57th Army. This part of Kyushu was considered that most likely to be invaded, and the plans third section provided for the defence of the Satsuma peninsula region by the 146th Division, 206th Division and 303rd Division, and the 125th Independent Mixed Brigade of the 40th Army. In the event of this area’s invasion, these formations and units were to attempt to check Major General Harry Schmidt’s V Amphibious Corps on the beaches until the mobile reserve could be assembled and moved from its inland locations. The counter-offensive phase in this sector was to be carried out by a mobile reserve comprising the 25th Division, 57th Division, 77th Division and 216th Division, together with the three tank brigades. The mobile reserve would advance to the area of Ijuin to seal off the Satsuma peninsula and prepare for the counter-offensive. There were also plans to redeploy two divisions from the 15th Area Army in the Kobe and Osaka areas of Honshu to augment the counter-offensive in southern Kyushu.
The 40th Army had a strong concentration of artillery and heavy mortars on the western side of Satsuma peninsula, to the south of Ijuin, in the 206th Division's sector, though this concentration was in fact closer to Fukiage Hama than to the beaches selected for the V Amphibious Corps' planned landing.Oddly enough, given the importance attached by the Japanese to the defence of Kyushu, many of the 40th Army's formations and units were considered in poor state of organisation and training. This was especially true of the 303rd Division and 206th Division.
However, the 77th Division was highly rated and came under the administrative control of the 40th Army and was held in reserve to the north of the Kagoshima Wan. This formation was to be prepared to support the 40th Army should the US forces make a landing on the western side of the Satsuma peninsula. The plan called for movement, chiefly on foot and by night, along the road beside the coast of the Kagoshima Wan and then across the peninsula on the road system just to the west of Kagoshima. The time estimated for this movement was six to seven days.
Another highly rated formation, the 25th Division was under the administrative control of the 57th Army, and was held in reserve in the area of Miyakonojo. It was ready to counterattack in the Miyazaki area, and was likewise to move chiefly on foot and by night, its estimated movement time being five days.
The 216th Division was located centrally in reserve at Kumamoto, ready to move to the south should the situation demand it. If the preliminary bombardment or early seizure of small islands to the south and south-west of Kyushu indicated the probability of an early invasion attempt on the southern part of Kyushu, the division was to move, again principally on foot and by night, to the area of Kirishima, to the north-west of Miyakonojo, in a period estimated at seven days. Similarly, if early indications pointed toward the invasion of the southern part of Kyushu, the 57th Division and 4th Independent Tank Brigade of the 56th Army were to be withdrawn from the area of Fukuoka area and moved by any and all methods available to the area of Kirishima.
The defensive plan also called for the use of the Civilian Volunteer Corps, which comprised not volunteers but all but medically exempted boys and men aged between 15 and 60 and all girls and women aged between 17 and 40. Armed largely with hand grenades, swords, sickles, knives, fire hooks and bamboo spears, these were to be led by regular forces and make extensive use of night infiltration patrols armed with light weapons and demolition charges.
The Japanese planned not to evacuate civilians or declare any open cities. The southern third of Kyushu in which the US invasion was expected had a population of 2.4 million persons in the 3,500 sq miles (9065 km²) of the Kagoshima and Miyazaki prefectures, but the defensive plan for the active defence of the few selected beach areas, and then to mass reserves for an all-out counter-offensive in the event that the invasion forces succeeded in securing a beach-head.
From this it is evident that the Japanese had decided to fight the final and decisive battle on Kyushu where, regardless of the cost, the Japanese military planned to repel any US landing attempt. The defence of the Japanese home islands was therefore centred on two primary operations: the employment of kamikaze craft of all types, and in far large numbers than had been used in earlier campaigns, against the invasion fleet’s transport vessels, and the army’s defence of the beaches.