Allied forces were prepared for mass casualties during Operation Downfall, but the atomic bombs prevented the plan from being used

Developing Operation Downfall

Following the success of D-Day, it became clear that victory in Europe was within reach—but the Pacific war still loomed large and unresolved.

In early 1945, the Allied leadership convened at the Argonaut Conference to map out the final steps toward defeating Japan. It was during this gathering that the blueprint for Operation Downfall—the proposed invasion of Japan—began to take shape.

Planners assumed that the war in Europe would wrap up by July 1, 1945, and that the ongoing Okinawa invasion would conclude by mid-August. Operation Downfall was divided into two major assaults: the first slated for November 1945, and the second in early 1946.

The initial wave would rely on troops already active in the Pacific, while the second would draw from forces shifted from Europe after Germany’s defeat. The planned invasion was monumental in scale, intended to exceed even the Allied landings at Normandy.

Operation Olympic

Operation Olympic, the first stage of the broader invasion strategy known as Operation Downfall, was slated to commence on November 1, 1945. Its objective was to seize control of Kyūshū, the southernmost of Japan’s main islands. Launching from Okinawa—which had recently been captured in a bloody and pivotal campaign—American forces would use the island as the primary staging ground for both Olympic and the second phase, Operation Coronet.

The scale of the invasion was unprecedented. Plans called for a vast armada of 400 destroyers and escort ships, 24 battleships, and 42 aircraft carriers that would join the effort. Fourteen divisions and two regimental combat teams would lead the ground assault, supported by a combined Allied fleet that included four Commonwealth battleships and 18 aircraft carriers.

To ensure air superiority and protect landing forces, the Fifth, Seventh, and Thirteenth U.S. Air Forces were tasked with providing cover and striking enemy defenses. Meanwhile, the U.S. Strategic Air Forces in the Pacific (USASTAF) and Britain’s Tiger Force prepared to conduct long-range bombing runs, with support from the legendary RAF No. 617 Squadron—the “Dambusters.”

Operation Coronet

After the expected success of Operation Olympic, the U.S. had a second invasion plan aimed at putting even more pressure on Japan by targeting Honshu. This next phase, called “Y-Day,” was scheduled for March 1, 1946, and was meant to bring the fighting even closer to Tokyo.

The plan involved landing 25 U.S. divisions from the First and Eighth Armies at Kujūkuri Beach and Hiratsuka. These forces would then be supported by 20 more divisions to help push deeper inland.

To strengthen the assault even further, five Commonwealth Corps divisions—made up of British, Canadian, and Australian troops—were added to the operation. Although they weren’t part of the original strategy, they were later included to help American forces surround Tokyo and advance toward Nagano.

Operation Downfall’s success would have come at a heavy price

While the Americans created a concrete plan for invading Japan, Operation Downfall would have come at a heavy price. As part of the preparation, they did calculations to see what the estimated casualties would be. While the numbers varied, the outcome was always staggeringly high.

One approach taken by Fleet Adm. William Leahy was that the casualties would be similar in number to those experienced on Okinawa – 35 percent. This meant the invasion of Kyūshū alone would have resulted in 268,000 casualties. This number was similarly echoed by Intelligence Chief Maj. Gen. Charles Willoughby.

Some estimates were far higher. One study conducted during deliberations showed that the invasion of Japan would cause up to four million American casualties and around 10 million Japanese deaths. It was these figures that played a role in the decision to drop the atomic bombs.

Operation Ketsugō

As the US planned its invasion, Japan was busy fortifying its defenses. Recognizing the increasing likelihood of an Allied assault, Japanese officials anticipated an attack post-1945 typhoon season. Remarkably, they accurately guessed the invasion locations.

Japan readied itself to counter 90 Allied divisions—20 more than what was actually expected. By this time, Japan understood that victory was out of reach. Instead, the strategy was to inflict such heavy costs on the invaders that the Allies might consider a truce.

Operation Ketsugō, Japan’s resistance strategy, not only involved significant military forces but also mobilized civilians. An extensive training program was put in place for new troops, including frogmen, and 28 million men and women were prepared for the Volunteer Fighting Corps. Additionally, Japan intended to use kamikaze pilots to stop the Allied naval forces from reaching the shores.

Downfall of Operation Downfall

Ultimately, Japan’s preparations were for nothing, as its troops couldn’t withstand the attacks that brought World War II to a close. Before Operation Downfall could be put into action, the Americans dropped the atomic bomb Little Boy on Hiroshima, followed shortly after by Fat Man over Nagasaki. The Japanese surrender and the Soviet advance into Manchuria that followed solidified for the allies that Downfall was no longer needed.

Prior to this, the US really was actually preparing for the invasion. The country even went so far as to create almost 500,000 Purple Hearts in advance of the waves of injured Americans that would come out of Operation Downfall. Since they were never needed, the US military opted to hand out these medals in future wars; so many were made, in fact, that they were given out during the Korean War, in Vietnam, and during the wars in Iraq and Afghanistan.

As of 2020, it was believed there could be as many as 60,000 of these Purple Hearts yet to be given out.