PUBLISHED on August 6, 2025, 11:21 AM EST – Key Points and Summary – Eighty years after the atomic bombing of Hiroshima, this analysis explores Operation Downfall, the massive Allied invasion of Japan that was averted by Tokyo’s surrender.
-The two-stage plan, Operations Olympic and Coronet, would have involved more than twice the forces of the Normandy landings and was expected to be unimaginably costly.
-Facing a fanatical defense, the U.S. anticipated staggering casualties, with some estimates exceeding one million American dead and unimaginable Japanese losses.
-While the use of the atomic bomb remains controversial, it likely spared the world from this far more devastating and bloody military conquest.
The Planned U.S. Invasion of Japan Was Twice the Size of Normandy.
Eighty years ago, the United States dropped an atomic bomb on the Japanese city of Hiroshima, inaugurating the nuclear age with the killing of some 100,000 people.
That event catalyzed the decision of the Empire of Japan to surrender two weeks later, ending the major action of the Second World War.
If Japan had not made that decision, history would have taken a different course.
Instead of celebrating VJ Day on August 15, we would likely commemorate the anniversaries of Operation Olympic (in November) and Operation Coronet (in March).
These twin invasions would have brought the war directly to Japan’s home islands. However horrific the use of the atomic bombs, the world was spared the far more dreadful outcome of a forcible conquest of Japan.
The Backstory and History: World War Coming to an End
The deliberations that led to Tokyo’s surrender in August 1945 remain clouded by bitter controversy in both Japan and the United States.
US historiography is preoccupied with the question of whether the atomic bomb was necessary, while questions of empire and war guilt have colored Japanese thinking.
Nevertheless, the most plausible accounts argue that the twin shocks of the atomic bombs and the Soviet invasion of Manchuria dislodged a stalemate in Japanese elite politics and enabled the emergence of a peace coalition, led by Emperor Hirohito himself.
An invasion only occurs in a world where this peace coalition does not emerge, either because of changes in external factors (such as the absence of an atomic bomb or a Soviet invasion) or because domestic factors unfold differently.
While this seems complicated to credit, the US continued to plan for both additional atomic attacks and a complete invasion in the days after the destruction of Nagasaki.
Indeed, some Japanese military officials resisted the surrender decision so bitterly that they attempted a coup in mid-August.
The Invasion Plans
Whatever the reason for Tokyo’s decision to continue the war, the Allies enjoyed a tremendous military advantage over Japan in 1945, especially after the surrender of Nazi Germany freed up forces assigned to Europe.
Japan’s military vulnerability left the Allies a great deal of latitude, with some senior decision-makers arguing that an invasion was unnecessary as blockade and bombardment would eventually force a Japanese surrender.
In part because the idea of victory through starvation sounded horrifying and in part because it would still leave the decision to end the war in Japanese hands, planning began as early as 1943 for Operation Downfall, an invasion of the home islands.
By 1945, these plans crystallized into a two-stage invasion to bring the war to an end. Operation Olympic would seize southern Kyushu following an amphibious assault in November 1945. Fourteen US divisions (more than twice the size of the Normandy invasion force) would have participated, supported by a massive air and sea armada.
Olympic would set the stage for Operation Coronet, an almost unimaginably large invasion of Honshu that would require some forty-five divisions in early 1946.
Controversy emerged around the participation of US allies in the invasion effort. The British Pacific Fleet (including French, Australian, Canadian, and New Zealand warships) would have supported Olympic.
A collection of Commonwealth forces (trained to American standards and using American weapons) would join Coronet, albeit only after considerable politicking in Allied command circles. For their part the Soviets contemplated an invasion of Hokkaido, although the lack of sufficient air and naval assets made the success of such an operation unlikely.
Expected Outcome
Japanese intelligence guessed accurately at the key elements of the Allied plan.
Although Japan lacked resources (especially energy), it had a considerable number of soldiers, a potentially fanatical civilian population, and bitter experience from defending a host of Pacific islands. Japanese planning envisioned a defense in depth supported by suicide attacks on the ground, in the air, and at sea.
While these preparations probably could not have prevented an Allied victory, they could have made it enormously costly. Indeed, Japanese preparations were made primarily for deterrence purposes, in the hopes that the Allies could be persuaded to accept an armistice in lieu of the invasion.
Casualty estimates for the invasion became suspect almost immediately because of political and organizational needs to justify particular policy decisions, especially the use of the atomic bombs.
Forecasts during the war ran from a hundred thousand US dead to over a million.
Japanese losses would simply have been unimaginable, as would damage to the home islands. Indeed, as additional atomic bombs became available, they would have been used against Japanese civilian and military targets, adding to the destruction.
Dropping Atomic Bombs…
The decision to use the atomic bombs is often justified by the specter of the costs of an invasion, and indeed, US policymakers did hope that the bombs would make an invasion unnecessary. Nevertheless, this paints the outcome in ways that are far too clean and concrete.
Both Americans and Japanese knew that the choice between atomic bomb and invasion was not clear-cut, and that the bombs might simply precede and to some extent pave the way for a bloody invasion.
In the worst-case scenario, Japan would have been subjected not only to atomic attacks but also to one of the most devastating military conquests in known history. Such an outcome would have had profound effects on the world today, with Japan left destroyed and the US potentially exhausted from another year of combat.
Fortunately, the military clique in Tokyo that would have continued the war did not get its way. The atomic bombs may or may not have been necessary (we will continue to argue this point for as long as there are Americans and Japanese), but Tokyo’s surrender in August 1945 saved the world the need to execute Operation Downfall.
About the Author: Dr. Robert Farley, University of Kentucky
Dr. Robert Farley has taught security and diplomacy courses at the Patterson School since 2005. He received his BS from the University of Oregon in 1997, and his Ph. D. from the University of Washington in 2004. Dr. Farley is the author of Grounded: The Case for Abolishing the United States Air Force (University Press of Kentucky, 2014), the Battleship Book (Wildside, 2016), Patents for Power: Intellectual Property Law and the Diffusion of Military Technology (University of Chicago, 2020), and most recently Waging War with Gold: National Security and the Finance Domain Across the Ages (Lynne Rienner, 2023). He has contributed extensively to a number of journals and magazines, including the National Interest, the Diplomat: APAC, World Politics Review, and the American Prospect. Dr. Farley is also a founder and senior editor of Lawyers, Guns and Money. You can reach him on X: @drfarls.
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