Operation Downfall
Operation Downfall was the proposed plan by United States and British Commonwealth forces for the invasion of the Japanese home islands near the end of World War II. It was canceled when Japan surrendered following the atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war, and the invasion of Manchuria.
The operation had two parts: Operation Olympic and Operation Coronet. Set to begin in November 1945, Operation Olympic was intended to capture the southern third of the southernmost main Japanese island, Kyūshū, with the recently captured island of Okinawa to be used as a staging area. In Spring 1946 would come Operation Coronet, the planned invasion of the Kantō Plain, near Tokyo, on the main Japanese island of Honshu. Airbases on Kyūshū captured in Operation Olympic would allow land-based air support for Operation Coronet. If Downfall had taken place, it would have been the largest amphibious operation in history, surpassing D-Day.
Japan's geography made this invasion plan obvious to the Japanese as well; they were able to accurately predict the Allied invasion plans and thus adjust their defensive plan,, accordingly. The Japanese planned an all-out defense of Kyūshū, with little left in reserve for any subsequent defense operations. Casualty predictions varied but were extremely high, from the low hundreds of thousands to over a million on the Allied side and into the millions for the Japanese.
Planning
Responsibility for the planning of Operation Downfall fell to American commanders Fleet Admiral Chester Nimitz, General of the Army Douglas MacArthur and the Joint Chiefs of Staff—Fleet Admirals Ernest King and William D. Leahy, and Generals of the Army George 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 the initial planning for the invasion of Japan did not take its existence into consideration. Once the atomic bomb became available, General Marshall envisioned using it to support the invasion if sufficient numbers could be produced in time.
The Pacific War was not under a single Allied commander-in-chief. Allied command was divided into regions: by 1945, for example, Chester Nimitz was the Allied C-in-C Pacific Ocean Areas, while Douglas MacArthur was Supreme Allied Commander, South West Pacific Area, and Admiral Louis Mountbatten was the Supreme Allied Commander, South East Asia Command. A unified command was deemed necessary for an invasion of Japan. Interservice rivalry over who it should be was so serious that it threatened to derail planning. Ultimately, the Navy partially conceded, and MacArthur was to be given total command of all forces if circumstances made it necessary.
Considerations
The primary considerations that the planners had to deal with were time and casualties—how they could force Japan's surrender as quickly as possible with as few Allied casualties as possible. Before the First Quebec Conference, a joint CanadianBritishAmerican planning team had produced a plan which did not call for an invasion of the Japanese Home Islands until 1947–48. The American Joint Chiefs of Staff believed that prolonging the war to such an extent was dangerous for national morale. Instead, at the Quebec conference, the Combined Chiefs of Staff agreed that Japan should be forced to surrender not more than one year after Germany's surrender.The United States Navy urged the use of a blockade and airpower to bring about Japan's capitulation. They proposed operations to capture airbases in nearby Shanghai, China, and Korea, which would give the United States Army Air Forces a series of forward airbases from which to bombard Japan into submission. The 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 prevailed.
Initially in early 1943, the Joint War Plans Committee and the Army's Strategy Section supported an invasion of Hokkaido via the Aleutians and Hawaii in 1945. They believed that Hokkaido was a well-placed, lightly defended island that offered suitable bases for a later invasion of Honshu. Planners eventually recognized however that the prospects for a successful invasion of Hokkaido in the summer of 1945 was increasingly unrealistic.
Physically, Japan made an imposing target, distant from other landmasses and with very few beaches geographically suitable for sea-borne invasion. Only Kyūshū and the beaches of the Kantō Plain were realistic invasion zones. The Allies decided to launch a two-stage invasion. Operation Olympic would attack southern Kyūshū. Airbases would be established, which would give cover for Operation Coronet, the attack on Tokyo Bay.
Assumptions
While the geography of Japan was known, the U.S. military planners had to estimate the defending forces that they would face. Based on intelligence available early in 1945, their assumptions included the following:- "That operations in this area will be opposed not only by the available organized military forces of the Empire, but also by a fanatically hostile population."
- "That approximately three hostile divisions will be disposed in Southern Kyushu and an additional three in Northern Kyushu at initiation of the Olympic operation."
- "That total hostile forces committed against Kyushu operations will not exceed eight to ten divisions and that this level will be speedily attained."
- "That approximately twenty-one hostile divisions, including depot divisions, will be on Honshu at the initiation of and that fourteen of these divisions may be employed in the Kanto Plain area."
- "That the enemy may withdraw his land-based air forces to the Asiatic Mainland for protection from our neutralizing attacks. That under such circumstances he can possibly amass from 2,000 to 2,500 planes in that area by exercising a rigid economy, and that this force can operate against Kyushu landings by staging through homeland fields."
Olympic
Olympic was also to include a deception plan, known as Operation Pastel. Pastel was designed to convince the Japanese that the Joint Chiefs had rejected the notion of a direct invasion and instead were going to attempt to encircle and bombard Japan. This would require capturing bases in Formosa, along the Chinese coast, and in the Yellow Sea area. However, as the actual build-up of American forces would clearly be directed towards Japan, the assault on China would then appear to be cancelled, with a new invasion seemingly being planned against Shikoku instead.
Tactical air support was to be the responsibility of the Fifth, Seventh, and Thirteenth Air Forces. These were responsible for attacking Japanese airfields and transportation arteries on Kyushu and Southern Honshu and for gaining and maintaining air superiority over the beaches. The task of strategic bombing fell on the United States Strategic Air Forces in the Pacific —a formation which comprised the Eighth and Twentieth air forces, as well as the British Tiger Force. USASTAF and Tiger Force were to remain active through Operation Coronet. The Twentieth Air Force was to have continued its role as the main Allied strategic bomber force used against the Japanese home islands, operating from airfields in the Mariana Islands. Following the end of the war in Europe in May 1945, plans were also made to transfer some of the heavy bomber groups of the veteran Eighth Air Force to airbases on Okinawa to conduct strategic bombing raids in coordination with the Twentieth. The Eighth was to upgrade their B-17 Flying Fortresses and B-24 Liberators to B-29 Superfortresses. In total, General Henry Arnold estimated that the bomb tonnage dropped in the Pacific Theater by USAAF aircraft alone would exceed 1,050,000 tons in 1945 and 3,150,000 tons in 1946, excluding the blast yields of nuclear weapons.
Before the main invasion, the offshore islands of Tanegashima, Yakushima, and the Koshikijima Islands were to be taken, starting on X−5. The invasion of Okinawa had demonstrated the value of establishing secure anchorages close at hand, for ships not needed off the landing beaches and for ships damaged by air attack.
Kyūshū was to be invaded by the Sixth United States Army at three points: Miyazaki, Ariake, and Kushikino. If a clock were drawn on a map of Kyūshū, these points would roughly correspond to 4, 5, and 7 o'clock, respectively. The 35 landing beaches were all named for automobiles: Austin, Buick, Cadillac, and so on through to Stutz, Winton, and Zephyr. With one corps assigned to each landing, the invasion planners assumed that the Americans would outnumber the Japanese by roughly three to one. In early 1945, Miyazaki was virtually undefended, while Ariake, with its good nearby harbor, was heavily defended.
The invasion was not intended to conquer the entire island, just the southernmost third of it, as indicated by the dashed line on the map labeled "general limit of northern advance". Southern Kyūshū would offer a staging ground and a valuable airbase for Operation Coronet.
After the name Operation Olympic was compromised by being sent out in unsecured code, the name Operation Majestic was adopted.