Truman's decision to drop the atomic bomb on Japan was absolutely the right decision. Not only did it stop Japan from becoming a threat to world peace, but it saved American lives. If he hadn't dropped the bomb, the war could have and would have dragged on for four or five more years.
Had Truman decided not to drop the bomb, Japan would have become a major threat to world peace. It would have become a bigger threat than it already was. Japan was a threat because it had more power than it knew what to do with. The Japanese regarded their Emperor as a god. They looked to him to tell them all the answers, as if he knew them all. They thought he knew exactly what to do at any given moment. (Doug Long: Hiroshima, Was It Necessary?) The Japanese would have invaded any country for any reason if the Emperor told them to do it.
This all means that Japan was a threat to world peace and Truman was justified in dropping the bomb on them.
When Truman decided to drop the bomb to save Allied lives, he was regarded highly among all the allies. Dropping the bomb on Japan saved what might have been thousands and thousands of Allied lives. This made the Allied leaders agree with his decision more than they did before. Before the Allies agreed with him they thought that dropping the bomb would too much of a massacre and they were no better than the Nazis. If they were no better than the Nazis, there would be no point in fighting the war. They might as well just pack up and let them win. In order to save Allied lives, Truman decided to drop the atomic bomb on Japan. (Doug Long:...