Well....was Harry Truman a war criminal? In my view, it is not even a close call. He was. If just war theory means anything it is that the intentional mass slaughter of civilians can not be justified. Had Hitler dropped an atomic bomb on London in 1941, judges at a subsequent war crimes trial would have dismissed out of hand any defense (even if it was partially true) that one goal was to"shorten the war and save lives from an invasion." They would have called it mass murder, pure and simple.
As Mark Brady points out, the entire basis of Whittle's argument falls apart once we abandon the premise of unconditional surrender (as first proclaimed by FDR). Truman rebuffed all proposals to let the Japanese keep the emperor in exchange for a surrender. A side benefit, of course, of such a conditional surrender would have been to avoid a bloody American invasion. Ironically, once Truman had dropped the bomb, he shifted course and agreed to this condition anyway. One of the best discussions of this issue is Thomas Fleming's magisterial, The New Dealers War.