President Truman had a very difficult decision to make in terms of dropping the atomic bomb. Before dropping the bomb, Truman was hoping to get an unconditional surrender from Japan after the demands made at the Potsdam Conference. However, Japan refused.
Instead of dropping the atomic bomb, he also considered an invasion of mainland Japan. However, this invasion was estimated to have millions of casualties total and would have resulted in the deaths of thousands of Allied forces and Japanese military personnel.
Ultimately Truman decides to drop bombs on Hiroshima and Nagasaki, effectively ending World War II. Even though this was a weapon of massive destruction, he argued that this actually saved lives. He used the estimated number of deaths/casualties from the possible land invasion to justify his decision.
