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Operation Downfall was the overall Allied plan for the invasion of Japan at the end of World War II. It was scheduled to occur in two parts—Operation Olympic, the invasion of Kyushu, set to begin in November, 1945; and later Operation Coronet, the invasion of Honshu near Tokyo, scheduled for the spring of 1946. Following the atomic bombing of Hiroshima and Nagasaki, and the Soviet declaration of war against Japan, the Japanese surrendered and the operation was cancelled.
Responsibility for planning the operation fell to the US commanders: Fleet Admiral Chester Nimitz, General of the Army Douglas MacArthur and the Joint Chiefs of Staff—Fleet Admirals Ernest King and William Leahy, and Generals of the Army George Marshall and Hap Arnold—who controlled the Twentieth Air Force, the strategic bombers. At the time, the development of the atomic bomb was a very closely guarded secret, known only to a few top officials, and planning for the invasion did not take its existence into consideration.
Throughout the Pacific War, unlike the European theatre, the Allies were unable to agree on a single Commander-in-Chief (C-in-C) in the Pacific. Allied command was divided into regions: by 1945, for example, Chester Nimitz was Allied C-in-C Pacific Ocean Areas, while Douglas MacArthur was Supreme Allied Commander, South West Pacific. For an invasion of Japan, a unified command was deemed necessary. Inter-service squabbling over who it should be—the US Navy wanted Nimitz, while the US Army wanted MacArthur—was so serious that it threatened to derail planning. Ultimately, the navy partially conceded and MacArthur was to have total command of all forces, if circumstances made it necessary.
The primary considerations that the planners had to deal with were time and casualties—how could they force Japan's surrender as quickly as possible, with as few Allied casualties as possible? In 1943, the Joint Chiefs agreed that Japan should be forced to surrender not more than one year after Germany's surrender. They were inspired to do this after seeing British-made plans that did not call for invasion of the home islands until the fall of 1947. Prolonging the war to such an extent was considered dangerous for national morale.
The US Navy urged the use of blockade and airpower to bring about Japan's capitulation. They suggested operations to capture airbases in nearby Shanghai, China and in Korea. These locations would give the US Army Air Force a series of forward airbases from which to operate against Japan. The US Army, on the other hand, argued that such a strategy could prolong the war indefinitely and expend lives needlessly, and therefore that an invasion was necessary. They supported mounting a large-scale thrust directly against the Japanese homeland, with none of the side operations that the Navy had suggested. Ultimately, the Army's viewpoint won out.
Physically, Japan made an imposing target. It has few beaches suitable for invasion. Only Kyushu (the southernmost island of Japan), and the beaches of the Kanto plain (both west of Tokyo, and north of Chiba) made suitable invasion zones. The allies decided to launch a two-pronged invasion. Operation Olympic would attack Kyushu in the South. Airbases would be established, and those would give cover for Operation Coronet, the attack on Tokyo Bay. This was obvious to the Japanese as well, who were able to accurately guess the strength and locations of the Allied invasions, and began to reinforce those areas as early as 1944.
Operation Olympic, the invasion of Kyushu, was to begin November 1, 1945. The combined Allied naval armada would have been the largest ever assembled, including forty-two aircraft carriers, twenty-four battleships and almost four hundred destroyers and destroyer escorts. Fifteen US divisions were scheduled to take part in the landing. Using Okinawa as a staging base, Olympic was to seize the southern portion of Kyushu, to use as a staging point. It was also to include a deception plan, known as Operation Pastel.
Kyushu was to be invaded at three points—Miyazaki beach, Ariake beach, and Kushikino beach. (If Kyushu were a clock, these would roughly correspond to 4, 5, and 7 o'clock, respectively). The invading Americans would outnumber the Japanese by roughly three to one. Miyazaki was virtually undefended, while Ariake (with its nearby good harbor) was heavily defended. However, the imposing terrain at Kushikino meant that the Marines who landed there would probably have the toughest time.
The invasion was not supposed to conquer the entire island, just the southernmost third of it (indicated by the dashed line on the map, "general limit of northern advance"). Southern Kyushu would become a staging ground for operation Coronet, and would give the Allies a valuable airbase from which to operate.
Because of a known buildup of Japanese troops on Kyushu leading up to the invasion, some American military planners proposed the use of chemical weapons prior to or during the invasion. Because of its predictable wind patterns and several other factors, Japan was particularly vulnerable to gas attack. Such attacks would neutralize the Japanese tendency to fight from caves—caves would only increase the soldiers' exposure to gas.
Although chemical warfare had been outlawed by the Geneva Protocol, neither the United States or Japan were signatories at the time. While the US had promised never to initiate gas warfare, earlier in the war Japan had used gas against the Chinese in Manchuria, (see Unit 731). This gave the US a reason for their use, and, as John Ray Skates notes,
In any amphibious operation, the defender has two choices for defensive strategy—strong defense of the beaches, or defense in depth. Early in the war (such as at Tarawa) the Japanese employed strong defenses on the beaches themselves, with little or no manpower in reserve. This tactic proved to be very vulnerable to pre-invasion shore bombardment. Later in the war, at Iwo Jima and Okinawa, the Japanese switched strategy and employed defense-in-depth. Fighting degenerated into attrition warfare. This led to very high casualties on both sides, to which the Americans were unaccustomed.
However, for the defense of Kyushu during Olympic, codenamed operation "Ketsu Go", the Japanese intended to revert to the tactic of defense on the beaches. The Japanese intended to repel the invasion, not just attrite it. (Once the invasion was repelled, they would be in a better position to negotiate for peace.) Using defense in depth, repelling the invasion would require troop mobility. Transit and communication systems (roads, rail, telephone) on Kyushu prior to the invasion were poor, and American airpower would destroy what little did exist very quickly. The only conceivable way the Japanese could repel the invasion would be to stop the American attack at the beaches.
Japanese defenses also would have relied heavily on kamikaze planes, which the Americans had grown to fear after losing thousands of navy personnel at the Battle of Okinawa. U.S. military intelligence estimated the number of Japanese aircraft to be around 2,000 aircraft; however this was only a fraction of the actual number available for kamikaze raids. U.S. military intelligence also estimated the numbers of possible defenders as rather low; in truth the Japanese had begun to organize a Home Guard to defend Kyushu from the American landings.
Operation Coronet, the invasion of Honshu at the Tokyo Plain south of the capital, was set to begin on December 1, 1945. This was later postponed to March, 1946. Coronet would have been the largest amphibious operation of all time, with fifteen divisions (including the floating reserve) earmarked for the operation.
Japanese defense was so committed to defending Kyushu against Operation Olympic that virtually no preparation was made to defend against Coronet.
Olympic was to be mounted with resources already in the theatre. If reinforcements had been needed, they could have been provided out of the forces being assembled for Coronet. However, Coronet would have needed the redeployment of substantial Allied forces from Europe, South Asia, Australasia and elsewhere.
Those would have included the US First Army and Eighth Air Force, which were in Europe. Army divisions, warships and air force units from Britain, Australia, India, Canada, New Zealand and elsewhere had been offered for use in the invasion. For example, "Tiger Force" was the name given to British Commonwealth squadrons, scheduled to be transferred from RAF Bomber Command in Europe to airbases on Okinawa, in support of the landings.
Given the Japanese predilection for fanatical resistance, the fact that Japanese civilians were being encouraged to become suicide attackers, and the large number of Japanese troops to be faced, high casualties were seen to be inevitable, but nobody knew with certainty how high. Several people made estimates but they varied widely in numbers, assumptions, and purposes—which included advocating various courses of action.
Everybody based their estimates on the experience of the preceding campaigns, but they could draw different lessons:
In a study done by the Joint Chiefs of Staff in April, the figures of 7.45 casualties/1000 man-days and 1.78 fatalities/1000 man-days, were developed. This implied that a 90-day Olympic campaign would cost 456,000 casualties, including 109,000 dead or missing. If Coronet took another 90 days, the combined cost would be 1,200,000 casualties, with 267,000 fatalities.
A study done by Adm. Nimitz's staff in May estimated 49,000 casualties in the first 30 days, including 5,000 at sea. A study done by Gen. MacArthur's staff in June estimated 23,000 in the first 30 days and 125,000 after 120 days. When these figures were questioned by Gen. Marshall, MacArthur submitted a revised estimate of 105,000, in part by deducting wounded men able to return to duty.
In a conference with President Truman on June 18, Marshall, taking Luzon as the best model for Olympic, thought the Americans would suffer 31,000 casualties in the first 30 days (and ultimately 20% of Japanese casualties, which implied a total of 70,000 casualties). Adm. Leahy, more impressed by Okinawa, thought the American forces would suffer a 35% casuality rate (implying an ultimate toll of 268,000). Adm. King, thought that casualties in the first 30 days would fall between Luzon and Okinawa, i.e. between 31,000 and 41,000.
Of these estimates, only Nimitz's included losses of the forces at sea, though in the Battle of Okinawa kamikazes had inflicted 1.78 fatalities/sortie, and the troop transports off Kyushu would be much more exposed. Moreover, all these estimates were done using intelligence that grossly underestimated Japanese strength being gathered for the battle of Kyushu in numbers of soldiers and kamikazes, by factors of at least three.
A study done for Navy Secretary Henry Stimson's staff by William Shockley estimated that conquering Japan would cost 1.7 – 4 million American casualties, including 400, – 800,000 fatalities, and five to ten million Japanese fatalities. The key assumption was large-scale participation by civilians in the defense of Japan.
Outside the government, well-informed civilians were also making guesses. Kyle Palmer, war correspondent for the Los Angeles Times, said half a million to a million Americans would die by the end of the war. Herbert Hoover, in memorandums submitted to Truman and Stimson, also estimated 500,000–1,000,000 fatalities. (But it is not known if Hoover discussed these specific figures in his meetings with Truman. The chief of the Army Operations division thought them "entirely too high" under "our present plan of campaign").
For context, the Battle of Normandy had cost 63,000 casualties in the first 48 days. The entire war cost America a total of just over a million casualties, with 300,000 fatalities.
Even if the atomic bomb had not been available, it is unlikely that Downfall—as planned—would have happened. On the Japanese side, it is likely that before the invasion began in November, with the loss of the delusion that the Soviet Union would help them mediate an armistice with the Allies, and with the prospect of mass famine and domestic unrest in the coming winter, even the Army leaders would have been compelled to accept the Allied terms of surrender. On the American side, it is likely that the prospective enormous casualties of an invasion of Kyushu would have led them to choose an alternate target such as Shikoku or northern Honshu, trading away the air support from Okinawa for the weakness of the local defending forces.
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