No, I do not agree with his decision to drop the atomic bomb. It was a terrible way to end the war, killing hundreds of innocent citizens and having lasting effects.
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In the greater context, Truman was absolutely correct.
As more Japanese would have died during an invasion, along with many thousands of US soldiers, the atomic bombs saved many other lives by ending the war. Their use also demonstrated reasons why they should never be used again. More Japanese civilians were killed by the conventional bombing campaign, and many more would have been.