Some of the major reasons that the United States dropped an atomic bomb on Hiroshima, Japan, include ending the World War II military activity in the Pacific as quickly and with as few American lives lost as possible, justifying the money spent on constructing the bomb, and showing the Soviet Union the might of American military power. Additionally, President Harry S. Truman decided to use the atomic bomb as revenge for Japan's attack on Pearl Harbor, Hawaii, in 1942.
Truman worried that an invasion of Japan would cost up to one million American lives and would drag on for far longer than the American public wanted due to the fanaticism of Japanese soldiers to defend their island and emperor. He also needed a reason to justify the $1,889,604,000 the government spent on the Manhattan Project, which in of itself was a secret program. He worried of the political ramifications of not using the weapon while spending almost $2 billion. Finally, Truman understood the Soviet Union's role in post-World War II global affairs would be one of an antagonistic player to American policies. He felt a showing of American military might would force the Soviet Union to reconsider its approach to its own Eastern European affairs.