Of course it was a war crime; Harry Truman was the war criminal. Truman would not accept anything less than an unconditional surrender from the Japanese, despite the fact that Japan was frantically making attempts to end the war in any way possible, short of an unconditional surrender. The Japanese were only concerned in maintaining the status of the Emperor. Despite numerous peace proposals, Truman was convinced that only an unconditional surrender would be sufficient.
The man who basically thought up the entire idea for an atomic bomb, Leó Szilárd, begged Truman's point man on nuclear matters, James Byrnes, to demonstrate the bomb's power on an empty target before turning it on civilians. Byrnes in a nutshell replied that all the money spent on creating the bomb could not be justified if it was not unleashed on Japanese cities. Truman gave the order that nuclear weapons should be used against Japan. Truman's decision clearly stemmed from hatred and bigotry of the Japanese people. Take a look at an excerpt from Truman's diary, dated July 25:
"We have discovered the most terrible bomb in the history of the world. It may be the fire destruction prophesied in the Euphrates Valley Era, after Noah and his fabulous Ark.
Anyway we "think" we have found the way to cause a disintegration of the atom. An experiment in the New Mexico desert was startling - to put it mildly. Thirteen pounds of the explosive caused the complete disintegration of a steel tower 60 feet high, created a crater 6 feet deep and 1,200 feet in diameter, knocked over a steel tower 1/2 mile away and knocked men down 10,000 yards away. The explosion was visible for more than 200 miles and audible for 40 miles and more.
This weapon is to be used against Japan between now and August 10th. I have told the Sec. of War, Mr. Stimson, to use it so that military objectives and soldiers and sailors are the target and not women and children. Even if the Japs are savages, ruthless, merciless and fanatic, we as the leader of the world for the common welfare cannot drop that terrible bomb on the old capital or the new.
He and I are in accord. The target will be a purely military one and we will issue a warning statement asking the Japs to surrender and save lives. I'm sure they will not do that, but we will have given them the chance. It is certainly a good thing for the world that Hitler's crowd or Stalin's did not discover this atomic bomb. It seems to be the most terrible thing ever discovered, but it can be made the most useful..."
The "warning statement" was basically the routine "surrender or be utterly destroyed." Not surprisingly, the Japanese didn't surrender. The bombs at Nagasaki and Hiroshima killed at least 140,000 people and injured over 100,000 people, not to mention the fact that tens of thousands more would die from injuries and radiation poisoning within two years.
On August 9, Truman delivered a speech to the American people about the use of the atomic bomb:
"The world will note that the first atomic bomb was dropped on Hiroshima, a military base. That was because we wished in this first attack to avoid, insofar as possible, the killing of civilians. But that attack is only a warning of things to come. If Japan does not surrender, bombs will have to be dropped on her war industries and, unfortunately, thousands of civilian lives will be lost. I urge Japanese civilians to leave industrial cities immediately, and save themselves from destruction."
It is rather hard to believe that Truman actually thought that Hiroshima was a military base, considering that 90% of the people killed were civilians.