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Why did U.S. leaders fear that invading Japan would cause heavy casualties?

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Atomic bombing of Hiroshima — atomic bombings of Hiroshima and Nagasaki A gigantic mushroom cloud rising above Hiroshima, Japan, on August 6, 1945, after a U.S. aircraft dropped an atomic bomb on the city, immediately killing more than 70,000 people.

U.S. leaders feared that invading the Japanese home islands would result in heavy casualties primarily due to the intense and costly resistance encountered during the Battle of Okinawa.1

The campaign for Okinawa, which served as a precursor to a potential invasion of Japan, was the longest and bloodiest in the Pacific theater. U.S. forces faced well-entrenched Japanese troops, leading to significant losses; American casualties included approximately 12,000 killed and 36,000 wounded. Furthermore, the Japanese military demonstrated a refusal to surrender, employing "last-resort" tactics such as kamikaze attacks and encouraging civilian suicide, which resulted in the deaths of roughly 100,000 Okinawan civilians.21

Based on these experiences, American planners estimated that a full-scale invasion of Japan could result in between 225,000 and 1,000,000 U.S. casualties. President Harry S. Truman sought to avoid repeating the devastation of Okinawa across the Japanese home islands, a concern that heavily influenced his decision to authorize the use of the atomic bomb.1