Allied Plans for the Invasion of Japan (Operation Downfall)
:: Japanese Homeland Dispositions August 1945 ::
In August 1945, as the Allies prepared for a possible invasion of the Japanese home islands (Operation Downfall), Japan's
military disposition reflected both desperation and determination.
The Japanese high command anticipated a two-phase Allied
assault: Operation Olympic targeting Kyushu (planned for November 1945) and Operation Coronet targeting Honshu (planned
for spring 1946). The Japanese defensive preparations were extensive but hampered by shortages in equipment, fuel, and
trained personnel.
Homeland Defense Forces
The core of Japan’s homeland defense was the Ketsugo strategy, which aimed to inflict massive casualties on invading
Allied forces to force a negotiated peace.
By August 1945, the Japanese had mobilized approximately 2.3 to 2.5 million regular army troops within the home islands.
This included both experienced combat units and large numbers of poorly trained reservists, conscripted students, and
members of the Patriotic Citizens Fighting Corps (composed of men aged 15–60 and women aged 17–40).
Defensive preparations included fortifications, beach obstacles, and stockpiling of kamikaze aircraft, suicide boats, and
human torpedoes for last-ditch attacks against landing forces.
The Imperial Japanese Navy was largely neutralized but retained some capacity for suicide missions and coastal defense.
The Japanese Kwantung Army, once the largest and most prestigious Japanese formation, was stationed in Manchuria
and was being depleted by August 1945 due to the transfer of units to defend the home islands.
In Korea, Japan maintained the Japanese Korean Army (Chōsen-gun). This force numbered several hundred thousand men,
tasked with internal security, suppressing resistance, and defending against possible Soviet or Allied incursions from the north.
The rapid Soviet advance into northern Korea beginning August 9, 1945, quickly overwhelmed Japanese forces, as the Soviets
moved southward before the Americans arrived in the south.
Strategic Context
The Japanese military command expected the main Allied invasion to come in Kyushu and concentrated their best remaining
divisions there, though many units were understrength and poorly equipped.
Civilian mobilization was at its peak, with widespread training in rudimentary combat and suicide tactics, reflecting the
regime’s willingness to sacrifice the population in defense of the emperor and homeland.
The disposition of forces in Korea and Manchuria was rendered moot by the Soviet declaration of war on August 8 and the
subsequent rapid Soviet advances.
:: Allied Plans for the Invasion of Japan (Operation Downfall) ::
Operation Downfall was the codename for the Allied plan to invade the Japanese home islands at the end of World War II.
It was conceived as a two-stage operation of unprecedented scale, planned to bring about Japan’s surrender through
direct military conquest.
The operation was ultimately rendered unnecessary by Japan’s surrender following the
atomic bombings of Hiroshima and Nagasaki and the Soviet declaration of war.
Operation Downfall consisted of two major phases:
Operation Olympic (Phase 1)
Operation Coronet (Phase 2)
Operation Olympic
Objective: Invade and secure the southern Japanese island of Kyushu.
Forces: Led by the U.S. Sixth Army.
Purpose: Establish airfields and naval anchorages on Kyushu to support the subsequent main invasion of Honshu.
Timeline: Planned for late 1945.
Scale: This would have been the largest amphibious invasion in history, surpassing the Normandy landings ("D-Day").
Operation Coronet
Objective: Invade the Kanto Plain near Tokyo, the industrial and political heartland of Japan.
Forces: Led by the U.S. First and Eighth Armies.
Purpose: Deliver the decisive blow to force Japanese surrender by capturing Tokyo and its surrounding region.
Timeline: Scheduled for spring 1946, after the establishment of bases on Kyushu.
Logistics: The success of Coronet was contingent on the capture of Kyushu in Olympic, which would provide air support
and staging areas for the larger assault on Honshu.
Strategic Considerations and Anticipated Costs
Planning began as early as 1943.
Casualty Estimates: Allied planners anticipated extremely high casualties, both military and civilian, due to Japan’s
demonstrated willingness to fight to the last man and the anticipated use of kamikaze tactics and mass mobilization
of the civilian population.
However, there was no singular consensus. For example, the
Joint War Plans Committee estimated around 130,000 U.S. casualties in the first 90 days of Olympic.
Estimates of other military and political sources (e.g., Stimson, Truman) ranged up to 500,000 – 1,000,000 U.S. casualties.
Japanese Defense: Japan’s counter-plan, Operation Ketsu-go, called for a "decisive battle" on the home islands, with
extensive use of kamikaze aircraft, fortified positions, and civilian resistance.
Operation Ketsu-go was Japan’s final defense plan against the anticipated Allied invasion of the home islands in 1945.
Its core objective was not to repel the invasion outright, but to inflict such massive casualties — using coordinated
Army, Navy, and Air Force attacks, including extensive kamikaze and suicide tactics by both military and civilian
forces — that the United States would reconsider its demand for unconditional surrender and instead negotiate peace
on more favorable terms for Japan.
Political Ramifications: The projected costs and devastation of Downfall played a significant role in the decision to
use atomic bombs, as the U.S. leadership sought to avoid the enormous losses expected from a conventional invasion.
Credits
Courtesy of the United States Military Academy Department of History.
