Final answer:
The U.S. military feared invading Japan because battles such as Iwo Jima and Okinawa indicated a potential invasion would result in severe casualties. The Japanese defense was determined and employed kamikaze tactics, and estimates predicted up to one million U.S. casualties. This led to the decision to use atomic bombs instead of invasion.
Step-by-step explanation:
The U.S. military feared invading Japan in the summer of 1945 primarily because the battles of Iwo Jima and Okinawa showed that such an invasion would result in an extremely high number of casualties on both sides. The Japanese defense was tenacious; for example, on Iwo Jima, out of approximately 25,000 Japanese defenders, only 200 survived. Okinawa exemplified the likely fierce resistance U.S. forces would face on the home islands, with 100,000 Japanese and 65,000 American casualties, including deaths and injuries.
Moreover, the Japanese employed tactics such as kamikaze attacks, which were essentially missions by pilots to cause maximum damage to U.S. naval forces. Estimates suggested that an invasion could last well into 1946, with U.S. casualties ranging from 500,000 to one million. The ferocity of the fighting and the extensive loss of life contributed to the decision to use the atomic bomb instead of a conventional invasion of Japan.