wikipedia gives a good concise argument. A supporter of Truman may argue one or two of these points, but when you take all of them into account, it is obvious that the bombings were done out of spite, and were war crimes....

http://en.wikipedia.org/wiki/Atomic_...a_and_Nagasaki

"The Manhattan Project had originally been conceived as a counter to Nazi Germany's atomic bomb program, and with the defeat of Germany, several scientists working on the project felt that the United States should not be the first to use such weapons. Two of the prominent critics of the bombings were Albert Einstein and Leo Szilard, who had together spurred the first bomb research in 1939 with a jointly written letter to President Franklin D. Roosevelt. Szilard, who had gone on afterwards to play a major role in the Manhattan Project, argued: "If the Germans had dropped atomic bombs on cities instead of us, we would have defined the dropping of atomic bombs on cities as a war crime, and we would have sentenced the Germans who were guilty of this crime to death at Nuremberg and hanged them." In the days just before their use, many scientists (including American nuclear physicist Edward Teller) argued that the destructive power of the bomb could have been demonstrated without the taking of lives.

The existence of historical accounts which indicate that the decision to use the atomic bombs was made in order to provoke an early surrender of Japan by use of an awe-inspiring power, coupled with the observation that the bombs were purposefully used upon targets which included civilians, has caused some commentators to state that the incident was an act of state terrorism. Historian Robert Newman, who is in favor of the decision to drop the bombs, took the claim of state terrorism seriously enough to argue that the practice of terrorism is justified in some cases.[37]

Some have claimed that the Japanese were already essentially defeated, and therefore use of the bombs was unnecessary. General Dwight D. Eisenhower so advised the Secretary of War, Henry L. Stimson, in July of 1945.[38] The highest-ranking officer in the Pacific Theater, General Douglas MacArthur, was not consulted beforehand but said afterward that he felt that there was no military justification for the bombings. The same opinion was expressed by Fleet Admiral William D. Leahy (the Chief of Staff to the President), General Carl Spaatz (commander of the U.S. Strategic Air Forces in the Pacific), and Brigadier General Carter Clarke (the military intelligence officer who prepared intercepted Japanese cables for U.S. officials);[38] Major General Curtis LeMay;[39] and Admiral Ernest King, U.S. Chief of Naval Operations, Undersecretary of the Navy Ralph A. Bard,[40] and Fleet Admiral Chester W. Nimitz, Commander in Chief of the Pacific Fleet.[41]

Eisenhower wrote in his memoir The White House Years:
"In 1945 Secretary of War Stimson, visiting my headquarters in Germany, informed me that our government was preparing to drop an atomic bomb on Japan. I was one of those who felt that there were a number of cogent reasons to question the wisdom of such an act. During his recitation of the relevant facts, I had been conscious of a feeling of depression and so I voiced to him my grave misgivings, first on the basis of my belief that Japan was already defeated and that dropping the bomb was completely unnecessary, and secondly because I thought that our country should avoid shocking world opinion by the use of a weapon whose employment was, I thought, no longer mandatory as a measure to save American lives."[42]
The United States Strategic Bombing Survey, after interviewing hundreds of Japanese civilian and military leaders after Japan surrendered, reported:

"Based on a detailed investigation of all the facts, and supported by the testimony of the surviving Japanese leaders involved, it is the Survey's opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945, Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war, and even if no invasion had been planned or contemplated."[43]
The survey assumed that continued conventional attacks on Japan—with additional direct and indirect casualties—would be needed to force surrender by the November or December dates mentioned.

In 1963 the bombings were subjected to judicial review in Shimoda et al. v. The State[44]. On the 22nd anniversary of the attack on Pearl Harbor, the District Court of Tokyo declined to rule on the legality of nuclear weapons in general, but found that "the attacks upon Hiroshima and Nagasaki caused such severe and indiscriminate suffering that they did violate the most basic legal principles governing the conduct of war."[45] In the opinion of the court, the act of dropping an atomic bomb on cities was at the time governed by international law found in the Hague Regulations on Land Warfare of 1907 and the Hague Draft Rules of Air Warfare of 1922–1923.[46] and was therefore illegal.[47]

Others contend that Japan had been trying to surrender for at least two months but the U.S. refused by insisting on an unconditional surrender. In fact, while several diplomats favored surrender, the leaders of the Japanese military were committed to fighting a "decisive battle" on Kyushu, hoping that they could negotiate better terms for an armistice afterward—all of which the Americans knew from reading decrypted Japanese communications. The Japanese government did not decide what terms, beyond preservation of an imperial system, they would have accepted to end the war; as late as August 9, the Supreme Council was still split, with the hardliners insisting Japan should demobilize its own forces, no war crimes trials, and no occupation. Only the direct intervention of the emperor ended the dispute, and even after that a military coup was attempted to prevent the surrender.

There is also some question as to how much Truman knew about the initial bombing. In his diary, he states that he told Secretary of War Stimson "to use it so that military objectives and soldiers and sailors are the target, not men, women, and children", and that they were in accord that "the target will be a purely military one".[48] Immediately after the bombing, Truman gave a speech stating, "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," seemingly not realizing that Hiroshima was a major city.[49][50]

Another criticism is that the U.S. should have waited a short time to gauge the effect of the Soviet Union's entry into the war. The U.S. knew, as Japan did not, that the Soviet Union had agreed to declare war on Japan three months after V-E Day; such an attack was indeed launched on August 8, 1945. The loss of any possibility that the Soviet Union would serve as a neutral mediator for a negotiated peace, coupled with the entry into combat of the Red Army (the largest active army in the world), might have been enough to convince the Japanese military of the need to accept the terms of the Potsdam Declaration (plus some provision for the emperor). Because no U.S. invasion was imminent, it is argued that the U.S. had nothing to lose by waiting several days to see whether the war could be ended without use of the atom bomb. As it happened, Japan's decision to surrender was made before the scale of the Soviet attack on Manchuria, Sakhalin Island, and the Kuril Islands was known, but had the war continued, the Soviets had plans to invade Hokkaido well before the Allied invasion of Kyushu.[51] Tsuyoshi Hasegawa's research has led him to conclude that the atomic bombings themselves were not the principal reason for capitulation. Instead, he contends, it was the swift and devastating Soviet victories on the mainland in the week following Stalin's August 8 declaration of war that forced the Japanese message of surrender on August 15, 1945.[52]

A number of organizations have criticized the bombings of Hiroshima and Nagasaki on moral grounds. To give one example, a 1946 report by the Federal Council of Churches entitled Atomic Warfare and the Christian Faith, includes the following passage:

"As American Christians, we are deeply penitent for the irresponsible use already made of the atomic bomb. We are agreed that, whatever be one's judgement of the war in principle, the surprise bombings of Hiroshima and Nagasaki are morally indefensible."