Operation Downfall

This was the US overall designation of plans for the final defeat of Japan by invasion of the home islands (1945).

‘Downfall’ was scheduled as a two-part undertaking whose first element would be the ‘Olympic’ invasion of Kyushu, scheduled to start in November 1945, and whose second element would be the ‘Coronet’ (ii) invasion of Honshu, near the Japanese capital city of Tokyo, scheduled to start in the spring of 1946. Following the ‘Silverplate’ and ‘Centerboard’ atomic bombings of Hiroshima and Nagasaki on 6 and 9 August 1945 respectively, and the Soviet declaration of war on the Japanese empire, Japan surrendered and further work on ‘Downfall’ and its components was cancelled.

The task of planning ‘Downfall’ was entrusted to the very highest levels of the US command structure in the form of Fleet Admiral Chester W. Nimitz, General of the Army Douglas MacArthur and the Joint Chiefs-of-Staff in the form of Fleet Admirals Ernest J. King and William D. Leahy, and Generals of the Army George C. Marshall and Henry H. Arnold. At the time, the development of the atomic bomb was a very closely guarded secret, known only to a few top officials outside the ‘Manhattan’ Project, and planning for the invasion therefore did not take its existence into consideration.

Throughout the Pacific campaign of World War II, unlike that in Europe, the Allies were unable to agree on a single commander-in-chief in the Pacific. The Allied command was therefore divided into regions: by 1945, for example, Nimitz was Allied commander-in-chief, Pacific Ocean Areas, while MacArthur was Supreme Allied Commander, South-West Pacific Area. For an invasion of Japan, a unified command was deemed necessary, however, and the inter-service squabbling over who this commander should be (the US Navy and US Army preferring Nimitz and MacArthur respectively) became so serious that there emerged a real threat to effective planning. Ultimately, the US Navy conceded, at least in part, and MacArthur was to exercise overall control if circumstances made it necessary.

The primary considerations with which the planners had to contend were time and cost (the latter considered in terms of casualties), translating as how best to encompass Japan’s defeat as swiftly as possible and with the minimum number of Allied casualties. In 1943, the Joint Chiefs-of-Staff had agreed that Japan should be forced to surrender not more than one year after the surrender of Germany. The Joint Chiefs-of-Staff were persuaded of this by the revelation that British planning did not envisage an invasion of the home islands until the autumn of 1947. The Joint Chiefs-of-Staff were sure, however, that the lengthening of the war against Japan to such an extent would be dangerous for national morale.

On the one hand, the US Navy urged the use of naval blockade and air power to bring about Japan’s surrender, and therefore recommended operations to take and hold areas in Korea and coastal China for the establishment of the air bases from which large numbers of USAAF strategic bombers could interdict Japan’s maritime lines of communication and destroy the nation’s war-making potential in the home islands.

On the other hand, the US Army was sure that such a strategy could and almost certainly would prolong the war indefinitely and result in the needless expenditure of lives, and therefore argued for a direct invasion. The US Army therefore supported a major thrust directly against the Japanese home islands with none of the side operations that the US Navy had suggested.

Ultimately it was the case of the US Army which prevailed. In purely physical terms, Japan was a large target but one with few stretches of coast suitable for amphibious landings. Only the southernmost major island, Kyushu, and the beaches of the Kanto plain (to both the south-west and south-east of Tokyo) on the island of Honshu offered suitable invasion zones. The Allies therefore decided to launch a two-stage invasion.

‘Olympic’ would be the first of these, and land in southern Kyushu. Here air bases would be built for the air forces whose support would ensure the success of the second landing, ‘Coronet’ (ii), the altogether more problematical landing in Tokyo Bay.

While the geography of Japan was known and was therefore a fully quantifiable factor, the planners could only estimate the forces which the Japanese defence might be able to deploy. Based on intelligence available early in 1945, the assumptions adopted by the planners included the fact that operations in this area would be opposed not only by the entirety of the available organised military forces of the Japanese empire, but also by a fanatically hostile population; that approximately three Japanese divisions would be disposed in southern Kyushu and another three divisions in northern Kyushu at the time ‘Olympic’ was launched; that the total Japanese forces committed against ‘Olympic’ would not exceed eight to 10 divisions and that this level would be attained quickly; that approximately 21 Japanese divisions, including depot divisions, would be based on Honshu at the time of the launch of ‘Coronet’ (ii) and that 14 of these divisions might be employed in the Kanto Plain area; and that the Japanese might withdraw their land-based air forces to the mainland of Asia for protection from US attempts to neutralise Japanese air strength on the home islands, and that under such circumstances the Japanese might thus be able to mass anything between 2,000 and 2,500 aircraft in these ‘safe areas’ for deployment against ‘Olympic’ by staging through air bases on the home islands.

The ‘Olympic’ invasion of Kyushu was scheduled for 1 November 1945, and the combined Allied naval armada would have been the largest ever assembled: it would have included 62 fleet, light and escort aircraft carriers, 24 battleships, 52 heavy and light cruisers, and more than 460 destroyers and destroyer escorts. Fourteen US divisions were scheduled to take part in the initial landings. Assembled and launched from Okinawa, already seized in ‘Iceberg’, and supported by the ‘Pastel’ (ii) deception plan, ‘Olympic’ was to seize the southern portion of Kyushu as a jumping-off point for ‘Coronet’ (ii).

Before the main invasion, two large offshore islands (Tanega-shima and Yaku-shima) as well as the all the smaller islands of the groups off the south-western end of Kyushu were to be taken from a time five days before ‘Olympic’: Tanega-shima was the target for 7,600 men of Brigadier General Hanford MacNider’s 158th Regimental Combat Team, while the Yaku-shima and Koshiki-retto were the targets for the 22,000 men of Brigadier General Donald J. Myers’s 40th Division. The reflected the US experience in ‘Iceberg’, which had proved the value of having secure anchorages close at hand for ships not needed off the landing beaches or damaged by air attack.

Kyushu was garrisoned by Lieutenant General Isamu Yokoyama’s 16th Area Army headquartered at Chikushino and possessing a strength of about 600,000 men: in the northern part of Kyushu was Lieutenant General Ichiro Shichida’s 56th Army with the 145th, 312th and 351st Divisions, 124th Independent Mixed Brigade, and as army reserves the 57th Division (20,000 men) and the 4th Tank Brigade; in the south-eastern part of Kyushu were the 150,000 men of Lieutenant General Nishihara Kanji’s 57th Army with the 109th Independent Mixed Brigade (5,900 men) on Tanega-shima, the 154th, 156th and 212th Divisions (55,000 men) in the area round Miyazaki, the 86th Division, 98th Independent Mixed Brigade, one independent infantry regiment and three independent infantry battalions (29,000 men) in the area round Ariake, and as army reserves the 25th Division and the 5th and 6th Tank Brigades; and in the south-western part of Kyushu were the 85,000 men of Lieutenant General Nakazawa Mitsuo’s 40th Army with the 303rd Division (12,000 men) at Sendai, 206th Division at Fukiage, the 146th Division, the 125th Independent Mixed Brigade on the southern part of the Satsuma peninsula), the 77th Division and one tank regiment and, as army reserves, the 216th Division and four independent mixed brigades.

The defence was also allocated some 5,000 aircraft as kamikaze machines with another 5,000 aircraft available for kamikaze service as and when required, 7,000 aircraft currently under repair, 100 ‘Koryu’ class midget submarines, 250 ‘Kairyu’ class midget submarines, 1,000 ‘Kaiten’ type manned torpedoes and 800 ‘Shinyo’ type suicide boats.

Kyushu was to be invaded by General Walter C. Krueger’s 6th Army at three points: Miyazaki (Major General Innis P. Swift’s I Corps comprising the 25th, 31st and 41st Divisions with 95,000 men), Ariake (Major General Charles P. Hall’s XI Corps comprising the 1st Cavalry Division and the 43rd and Americal Divisions with 113,000 men), and Kushikino (Major General Harry Schmidt’s V Amphibious Corps comprising 2nd, 3rd and 5th Marine Divisions with 99,000 men). The 35 landing beaches were all named for makes of automobile from ‘Austin’ to ‘Zephyr’, and the 6th Army’s reserves were Major General Charles W. Ryder’s IX Corps (77th, 81st and 98th Divisions) with 79,000 men) and Major General Joseph Swing’s 11th Airborne Division with 15,000 men.

The assault forces were to be transported and landed by Admiral Thomas C. Kinkaid’s 7th Fleet, with escort and longer-range support provided by Admiral William F. Halsey’s 3rd Fleet (20 fleet and light carriers, nine modern battleships, 26 super-heavy, heavy, light and anti-aircraft light cruisers, and 75 destroyers), and close escort and assault support provided by Admiral Raymond A. Spruance’s 5th Fleet (36 escort carriers, 11 battleships, 26 heavy and light cruisers, 387 destroyers and destroyers escorts, 394 assorted assault ships and 977 assorted landing ships). Admiral Sir Bruce Fraser’s British Pacific Fleet would also provide six fleet carriers to boost the 3rd Fleet.

Air support of all types was to be entrusted to General George C. Kenney’s 119,000-man Far East Air Forces (5th, 7th and 13th AAFs with 14 bomber and 10 fighter groups), General Carl A. Spaatz’s (later Major General Curtis E. LeMay’s) Strategic Air Forces (Major General Nathan F. Twining’s 77,000-man 20th AAF with 1,000 Boeing B-29 Superfortress heavy bombers, and Lieutenant General James H. Doolittle’s 8th AAF, the British ‘Tiger’ Force (detached from RAF Bomber Command) with 480 to 580 Avro Lancaster heavy bombers (about half to be used as tankers), and Group Captain Charles Read’s Australian 1st Tactical Air Force with 20 fighter and attack squadrons.

With one three-division corps assigned to each of the three landing areas, the planners assumed that the US forces would outnumber the Japanese on about a 3/1 basis.

Early in 1945 Miyazaki was virtually undefended while Ariake, with its nearby good harbour, was heavily defended. Although Kushikino was weakly defended, its imposing terrain meant that the US Marines who landed there would probably have the hardest time.

The invasion was not intended to preface a conquest of the whole of Kyushu, but rather to seize and hold its southernmost third. Southern Kyushu would offer a staging ground and a valuable air base for ‘Coronet’ (ii).

The ‘Coronet’ (ii) invasion of Honshu, on the shores of the Kanto Plain to the south of Tokyo, was scheduled for 1 March 1946, and would have been the largest amphibious operation of all time, with 25 divisions (including the floating reserve) earmarked for the initial operations against General Doihara Kenji’s 12th Area Army (36th, 51st, 52nd and 53rd Armies with 18 infantry and two armoured divisions) and its headquarters at Tokyo.

General Courtney H. Hodges’s 1st Army would have landed Lieutenant General Roy S. Geiger’s III Amphibious Corps (1st, 4th and 6th Marine Divisions) and Lieutenant General John R. Hodge’s XXIV Corps (7th, 27th and 96th Divisions), at Kujukuri on the Boso peninsula, with the 5th, 44th and 86th Divisions as the follow-on corps, while Lieutenant General Robert L. Eichelberger’s 8th Army would have landed Major General Franklin D. Sibert’s X Corps (24th, 31st and 37th Divisions), Lieutenant General Oscar W. Griswold’s XIV Corps (6th, 32nd and 38th Divisions), and Major General Alvan C. Gillem’s XIII Corps (13th and 20th Armored Divisions), at Hiratsuka on Sagami Bay, with the 4th, 8th and 87th Divisions as the follow-on corps.

Both armies would then have driven to the north and inland, meeting at Tokyo.

Other forces available for ‘Coronet’ (ii) would have been the Army Forces Pacific Reserve in the form of the 97th Division and 11th Airborne Division as well as two follow-on corps (2nd, 28th and 35th Divisions in one, and the 91st, 95th and 104th Divisions in the other), and Lieutenant General C. F. Keightley’s Commonwealth Corps (British 3rd, Canadian 6th and Australian 10th Divisions).

It was planned that ‘Olympic’ would be mounted with resources already in the Pacific theatre.

As noted above, these included Fraser’s British Pacific Fleet, which was actually a British and commonwealth formation with at least a dozen aircraft carriers and several battleships, and the 20-squadron Australian 1st Tactical Air Force, which had taken part in the invasion of the Philippine islands group in October 1944. These would have probably augmented US close air support units over Japan.

The only major redeployment involved in ‘Olympic’ was ‘Tiger’ Force, the name given to a group of more than 30 British and commonwealth bomber squadrons, scheduled to be transferred from the RAF’s Bomber Command in Europe to air bases on Okinawa.

If reinforcements had been needed, they could have been provided from forces being assembled for ‘Coronet’ (ii), which would have needed the movement of substantial Allied forces from Europe, South Asia, Australasia and elsewhere. These would have included the US 1st Army (15 divisions) and the US 8th AAF, which were both in Europe. The redeployment was complicated by the simultaneous partial demobilisation of the US Army after the surrender of Germany, which drastically reduced the divisions’ combat effectiveness by stripping them of their most experienced officers and men. Keightley’s Commonwealth Corps, initially comprising single British, Australian and Canadian infantry divisions, was being formed for use in ‘Coronet’ (ii). Major reinforcements would have been available from those countries, as well as other parts of the commonwealth. MacArthur had rejected proposals to include an Indian division, ostensibly as a result of language issues. He was also pushing for the Commonwealth Corps to be organised on US army lines and to use predominantly US equipment to simplify logistics.

Over this same period, the Japanese had been laying their own plans to counter what they now believed was the inevitable Allied invasion of their home islands. At first they were concerned by the possibility of such an operation as early as the summer of 1945. The fighting for Okinawa lasted so long, however, that they came to the conclusion that the Allies would not be able to launch another major operation before the advent of that year’s typhoon season, during which the weather would be too risky for amphibious operations.

Japanese intelligence predicted fairly closely where the invasion would take place: southern Kyushu at Miyazaki, Ariake Bay and/or the Satsuma peninsula.

While Japan no longer had a realistic prospect of winning the war, the Japanese high command believed that it could perhaps raise the cost of any Allied conquest to a point at which the Allies would decide the price was too high, perhaps paving the way for an armistice.

The Japanese plan for defeating the invasion was ‘Ketsu’ (decisive), and this relied heavily on the employment of kamikaze aircraft. In addition to fighters and bombers, the Japanese reassigned almost all of their training aircraft for the mission in an effort to exploit quantity as an alternative to the quality they no longer possessed. Their army and navy had more than 10,000 aircraft ready for use in July, and would have had a somewhat higher number by October, and the high command was planning to expend nearly this whole total against the invasion fleets.

During the fighting for Okinawa, fewer than 2,000 kamikaze aircraft had achieved one hit for every nine attacking aircraft. Off Kyushu, given that the kamikaze aircraft would be operating under more favourable circumstances, the Japanese hoped to get one hit for every six aircraft, and therefore estimated that the kamikaze aircraft would sink more than 400 ships. Moreover, the Japanese were now training their pilots to target transport ships rather than warships, and thus expected US personnel casualties to be disproportionately greater than they had been off Okinawa. One staff study estimated that the kamikaze aircraft could destroy between one-third and one-half of the invasion force before it could undertake its landings.

The Japanese navy had by this time ceased to be an effective fighting force. By August the service possessed only six carriers, four cruisers and one battleship, none of which could be adequately fuelled, and could keep an operational force of perhaps 20 destroyers and 40 submarines at sea, but only for a few days. The navy did, on the other hand, possess about 100 ‘Koryu’ type midget submarines, 250 smaller ‘Kairyu’ type midget submarines, and 1,000 ‘Kaiten’ type manned torpedoes. The army also had 800 ‘Shinyo’ type suicide boats.

The Japanese also considered the possibility that their kamikaze forces would not be able to halt the Allied advance before its landed its troops. In seeking to defeat an amphibious landing, the defence has the options of a strong defence of the assault beaches, or a layered defence in greater depth. Early in the war, in operations such as that at Tarawa atoll, which the Americans took in ‘Galvanic’ during November 1943, the Japanese employed strong beach defences, with little or no manpower in reserve. This tactic proved to be very vulnerable to pre-invasion shore bombardment. Later in the war, at Peleliu (‘Stalemate II’), Iwo Jima (‘Detachment’) and Okinawa (‘Iceberg’), the Japanese switched to the other tactic and dug their forces into the most defensible terrain, whereupon the fighting became attritional, with very high US casualties but no hope of a Japanese victory.

For the defence of Kyushu, the Japanese adopted an intermediate tactic, with the bulk of their defensive forces a short distance inland, immune to naval gunfire but close enough that the US forces would not be able, the Japanese hoped, to establish a secure beach-head before the Japanese moved forward to check them. The counter-offensive forces were still farther back, prepared to move against whichever of the landings seemed to be developing as the main effort.

In March 1945, there was only one combat division in Kyushu. Over the next four months the Japanese army transferred forces from Manchukuo, Korea and northern Japan, while raising other forces in place. By August, therefore they had 14 divisions and various smaller units, including three tank brigades, with a total of 900,000 men. The Japanese were able to conscript new soldiers in large numbers, but the provision of their equipment and weapons was an altogether more difficult task. By August, the Japanese army had the equivalent of 65 divisions in the home islands, but equipment for only about 40 of them, and only enough ammunition for 30.

The Japanese did not formally decide to stake everything on the outcome of the battle for Kyushu, but did concentrate their main assets one that island to such a degree that there would be little left in reserve. By one estimate, the forces in Kyushu had 40% of all the ammunition in the home islands.

In addition, the Japanese had organised nearly all adult civilians into the Patriotic Citizens Fighting Corps to perform combat support and, ultimately, combat tasks. These militia forces generally lacked training and weapons, but they were expected to make do with what they had.

The US military intelligence apparatus initially estimated the number of Japanese aircraft to be around 2,500. The Okinawa experience was bad (almost two fatalities and a similar number wounded per sortie), and it was expected that the fighting on Kyushu would be more costly. To attack the ships off Okinawa, the kamikaze aircraft had to fly long distances over open water, but for attacks on US shipping off Kyushu they could fly overland and then only the shortest of overwater distances to the landing fleets. Then US intelligence gradually discovered that the Japanese were devoting all their aircraft to the kamikaze mission, and taking effective measures to conserve them until the battle. A US Army estimate in May 1945 was 3,391 aircraft, in June 4,862 and in August 5,911. A US Navy estimate, omitting any distinction between training and combat aircraft, was 8,750 in July and 10,290 in August.

The Allies made counter-preparations, replacing some of their aircraft carriers’ torpedo-bomber and dive-bomber squadrons with more fighter squadrons, and converting obsolescent Boeing B-17 long-range bombers into airborne radar pickets. Nimitz came up with a plan for a pre-invasion feint, sending a fleet to the invasion beaches a couple of weeks before the real invasion, to lure out the Japanese on their one-way flights only to find not the valuable but vulnerable transport ships they expected, but warships loaded with anti-aircraft guns from stem to stern.

Throughout April, May and June, Allied intelligence followed the build-up of Japanese ground forces, including five divisions added to Kyushu’s forces, with great interest but some complacency, still projecting that in November the total for Kyushu would be about 350,000 troops. That changed in July, with the discovery of four new divisions and indications of more to come. By August, the count was up to 600,000, and ‘Magic’ Sigint had identified nine divisions in southern Kyushu: this was three times the expected number, and was still a serious underestimation of the real Japanese strength.

The intelligence revelations about Japanese preparations on Kyushu emerging in mid-July came as a considerable shock in both the Pacific and Washington. On 29 July, MacArthur’s intelligence chief, Major General Charles A. Willoughby, noted first that the April estimate allowed for the Japanese capability to deploy six divisions on Kyushu, with the potential to deploy 10, then added that ‘These [six] divisions have since made their appearance, as predicted, and the end is not in sight.’ If not checked, this threatened ‘to grow to [the] point where we attack on a ratio of one (1) to one (1) which is not the recipe for victory.’

The build-up of Japanese troops on Kyushu led US planners, most importantly Marshall, to consider drastic changes to ‘Olympic’, or its replacement by a different plan. As a result of its very predictable wind patterns and several other factors, Japan was particularly vulnerable to gas attack. Such attacks would neutralise the Japanese predilection to fight from caves, for these enclosures would only increase the soldiers’ exposure to gas. Although chemical warfare had been outlawed by the Geneva Protocol, neither the USA nor Japan was a signatory at the time in question. While the USA had declared its intention never to initiate gas warfare, earlier in the war Japan had used gas against the Chinese in Manchuria. This gave the USA any excuse it might need.

On Marshall’s direct instruction, Major General John E. Hull, Assistant Chief-of-Staff in the War Department General Staff, looked into the tactical use of atomic weapons for the invasion of the Japanese home islands. (Even after dropping two strategic atomic bombs on Japan, Marshall did not immediately believe that the Japanese would capitulate.) Colonel Lyle E. Seeman reported that at least seven bombs would be available by X-day for delivery onto the defending forces. Seeman advised that US troops not enter an area hit by a bomb for ‘at least 48 hours’: the risk of fall-out and residual radiator was not well-understood at the time, and so short an interval would in fact have resulted in substantial radiation exposure for the US troops.

The planners of the Joint Chiefs-of-Staff, noting the extent to which the Japanese had concentrated their defensive thinking on Kyushu at the expense of the rest of Japan, also considered alternative invasion areas, these including the island of Shikoku to the south of the western tip of Honshu, or the northern end of Honshu at Sendai or Ominato, or even the skipping of the preliminary invasion in favour of a direct assault on Tokyo. Attacking northern Honshu would have the advantage of a meeting a considerably weaker defence, but at the cost of giving up air support (except the Boeing B-29 Superfortress heavy bombers) from Okinawa.

MacArthur dismissed any need to change his plans. ‘I am certain that the Japanese air potential reported to you as accumulating to counter our OLYMPIC operation is greatly exaggerated…As to the movement of ground forces…I do not credit…the heavy strengths reported to you in south Kyushu…In my opinion, there should not be the slightest thought of changing the OLYMPIC operation.’

King, the Chief of Naval Operations, was prepared to state his official opposition to the invasion, with Nimitz’s concurrence, which would have set off a major dispute within the US high command. At this juncture, the key interaction would likely have been between Marshall and President Harry S Truman. There is strong evidence that Marshall remained committed to an invasion as late as 15 August, but tempering Marshall’s personal commitment to invasion would have been his comprehension that civilian approval in general, and Truman’s in particular, was unlikely for a costly invasion that no longer enjoyed consensus support from the armed services.

Unknown to the Americans, the Soviets were preparing to follow up their invasions of Sakhalin and the Kurile islands group toward the end of 'Avgust Buri' with an invasion of the weakly defended northern island of Hokkaido by the end of August, which would have put pressure on the Allies to do something sooner than November.

On 15 August, though, the Japanese agreed to surrender, rendering moot the whole question of invasion.

Given the fanatical nature of Japanese defensive fighting, as revealed in most previous campaigns, the fact that Japanese civilians were being encouraged to become suicide attackers, and the large number of Japanese troops to be faced, the Americans believed that high casualties would be inevitable, but what no one knew was how high these casualties might be. Several studies were initiated, but no consensus was reached on method and thus different conclusions were drawn. For example, in a study by the Joint Chiefs-of-Staff in April 1945, the figures of 7.45 casualties per 1,000 man-days and 1.78 fatalities per 1,000 man-days emerged: from this is was concluded that the proposed 90-day ‘Olympic’ campaign would cost 456,000 casualties, including 109,000 dead or missing, and if ‘Coronet’ (ii) took another 90 days, the combined cost would be 1.2 million casualties, including 267,000 dead.

A study by Nimitz’s staff in May estimated 49,000 casualties in the first 30 days, including 5,000 at sea, while another by MacArthur’s staff in June estimated 23,000 casualties in the first 30 days and 125,000 after 120 days. When these figures were questioned by Marshall, MacArthur submitted a revised estimate of 105,000 casualties, in part by deducting wounded men able to return to duty.

In a conference with Truman on 18 June, Marshall took the fighting for Luzon island in the Philippine islands group as the best available indicator for ‘Olympic’, and suggested that the US forces would suffer 31,000 casualties in the first 30 days (and ultimately 20% of Japanese casualties, which implied a total of 70,000 casualties). Leahy, more impressed by Okinawa, thought the American forces would suffer a 35% casualty rate (implying an ultimate toll of 268,000). King thought that casualties in the first 30 days would fall between those of Luzon and Okinawa, that is between 31,000 and 41,000. Of these estimates, only that of Nimitz included losses of the forces at sea, though in the fighting for Okinawa kamikaze aircraft had inflicted 1.78 fatalities per kamikaze pilot, and the troop transports off Kyushu would be much more exposed.

Moreover, all these estimates were generated on the basis of intelligence that grossly underestimated (by factors of at least three) the Japanese strength being gathered for the battle of Kyushu in numbers of soldiers and kamikaze aircraft. A study done for Secretary of War Henry L. Stimson’s staff by the physicist and analyst William B. Shockley estimated that the conquest of Japan would result in between 1.7 and 4 million US casualties, including some 400,000 to 800,000 fatalities, and between 5 and 10 million Japanese fatalities. The key assumption was large-scale participation by civilians in the defence of Japan.