Why did truman drop an atomic bomb on japan?

To end the war quickly, to save up to 500,000 (and some say up to a million) GI and Japanese lives, and to prevent Stalinization of the Orient, similar to the Eastern European Bloc. Those are the myths or justifications we fall back on to justify their use.

but when you think about it, we did more damage with the incendiary attacks from Feb 45 to August 45 than the 2 a-bomb attacks. To me, dead is dead, and the barbarity of war is always there, whether you inpale one baby on the end of a bayonet or obliterate tens of thousands in a few seconds....dead is dead.

Those who try to moralise Truman's decision in dropping the bomb are the ones most likely to be defeated in wars....that to me is the lesson of Dresden, Hiroshima and Nagasaki...