There are many reasons. This will always be a controversial topic. So, Truman had a choice, continue the bloodiest campaign in the pacific and risk losing thousands of more soldiers, or use the atomic bombs and end the war. The problem? The victims of the bombings were civilians, NOT soldiers. When he used the Uranium bomb on Hiroshima, it was devastating but Japan did not surrender. So, they bombed Nagasaki with a Plutonium bomb. Japan was afraid they might use a third bomb, so they surrendered. These two atomic bombs were the only two atom bombs ever used in war.