Japan herself was not an innocent victim of unnecessary American aggression. She had invaded Manchuria in 1931 and then the rest of China in 1937. Her soldiers had committed countless atrocities against the Chinese people and later the Allies. She used Allied prisoners of war in biological weapons experiments at the infamous “Unit 731” in Manchuria. Had the Allied invasion of the Home Islands come, the High Command had issued order for all Allied POW’s to be executed. It was well known among the POW’s that the Japanese were preparing to execute them. For them, the atomic bombs were a divine deliverance. Japan waged aggressive, bloody, and murderous war starting in 1931 and it took the power of the United States more than 3½ years to finally end it.
One must also not forget American attitude toward the Japanese. Japan, without a declaration of war, attacked the United States on December 7th, 1941. This created an attitude of resentment and hatred that guaranteed that the war in the Pacific would be a war to the death.
While Truman and his advisors were primarily concerned with American casualties, the atomic bombs also prevented an even larger number of casualties among the Japanese themselves. With millions of Japanese on the Home Islands, there can be no doubt that the Japanese would have suffered an almost unimaginable number of dead and wounded. Continual firebomb raids, naval bombardment, and the possible use of chemical weapons all could have pushed the number of Japanese casualties into the millions. It was for more merciful to drop two atomic bombs and kill between 150,000 and 250,000 than to kill millions. If these cities had not been atomic targets, they would have been burned to the ground by LeMay’s Superfortresses probably killing just as many people if not more.
"To avert a vast, indefinite butchery, by a few explosions seemed, after all our perils and toils, a miracle of deliverance." - Winston Churchill