In August of 1945, the American military was preparing plans for the upcoming invasion of mainland Japan. The Japanese had been pushed all the way across the Pacific; their war industry had been smashed; and they were starving. They still controlled parts of New Guinea, the Dutch East Indies, and Indochina where they had massive numbers of troops. Yet, their forces there were mostly cut off and had no hope of coming to the assistance of the Japanese islands.

After defeats in bloody, terrible fighting in the Philippines, the Marshall Islands, Iwo Jima, and Okinawa the U.S. was at Japan’s doorstep. B-29 bombers could now hit Japanese cities and were also close enough to have fighter cover all the way to Japan and back.

Yet, those American victories had been very costly. Whereas Japanese casualties had been five to six times that of Americans earlier in the war, by the time Okinawa was finally secured in June 1945, the ratio had dropped to two to one as Japanese troops would rather fight to the death than surrender. Further, since the battle of the Philippine Sea, Japanese suicide pilots (kamikaze) had also been taking a toll on American warships and lives.  

Nevertheless, Japan wasn’t yet defeated and the Japanese were rushing troops back from China and creating more homeland divisions.

A Difficult Decision and the Choice of Targets

Fearing that an invasion of Japan would cost hundreds of thousands of U.S. casualties and millions of Japanese, the decision by President Truman to drop the atomic bomb on Japan was not made lightly.

Proposals of using poison gas on the Japanese or dropping the atomic bomb with a warning in an unpopulated area were rejected. Nevertheless, there were still some doubts about whether the bombs would even work.

The plutonium bomb “Fat Man” is loaded on a trailer before being dropped on Nagasaki. (National Archives)

The U.S. created the 509th Composite Group, commanded by Colonel Paul Tibbetts, to conduct the drops.

The components for the bomb were delivered by the USS Indianapolis to Tinian on July 26. By early August, the components for the bomb were coming together. Fearing that a crash upon takeoff would result in a nuclear explosion, Tibbetts had the engineers modify the Little Boy bomb design to incorporate a removable breech plug that would permit the bomb to be armed in flight.