The US bombed Hiroshima and Nagasaki

For
1. It was necessary to shorten the war. A full scale invasion of the Japanese main land, would cost the lives of a lot of American soldiers, and using the atomic bombs as a threat of total destruction was a more favourable strategy.

2. The US bombed Hiroshima and Nagasaki, showed the Japanese the error of their ways, and have paid them reparations and helped rebuild the whole country after the war. It was fair that Americans bomb Japan in return.

3. The Manhattan Project expense required use of the bomb. The Manhattan Project was the most expensive government project in history at that time. By the time Roosevelt died in April 1945, almost 2 billion dollars of taxpayer money had been spent on the project. The President's Chief of Staff, Admiral Leahy, said, "I know Roosevelt would have used it in a minute to prove that he had not wasted $2 billion.”

Against
1. The atomic bomb was built as a defensive weapon, not an offensive one. It was intended to be a deterrent, to make Germany or any other enemy think twice before using such a weapon against the United States. The bomb was made for defence only; they should never have been used offensively in the first place.

2. The atomic bomb use as a weapon was simply inhumane. Entire families, whole neighborhoods were simply wiped out, people were vaporized, horrifically burned, buried in rubble, speared by flying debris, and saturated with radiation because of the atomic bomb used. Also survivor need to faced radiation sickness, starvation, and crippling mutilations, and psychological damage, even some people suicide because they unable to cope with trauma left untreated. Japanese of Hiroshima and Nagasaki were shunned and ostracized from Japanese society. Radiation continued to haunt the survivors, bringing a lifetime of sickness, not the least of which was an increase in the rates of various cancers. Birth defects for those pregnant at the time jumped significantly, and although the data on birth defects passed down through generations is inconclusive (Hiroshima and Nagasaki are ongoing laboratories of the long-term effects of radiation exposure), bomb survivors and their offspring continue to suffer anxiety about the possibilities.

3. Use of the bomb was illegal. On September 39, 1938, the League of Nations, "under the recognized principles of international law,” issued a unanimous resolution outlawing the intentional bombing of civilian populations, with special emphasis against bombing military objectives from the air. The League warned, "Any attack on legitimate military objectives must be carried out in such a way that civilian populations in the neighbourhood are not bombed through negligence." Significantly, the resolution also reaffirmed that "the use of chemical or bacterial methods in the conduct of war is contrary to international law." In other words, a special category of illegal weapons had been recognized, a category today called Weapons of Mass Destruction (WMD)

My opinion is US should have used the atomic bombs. Because US wanted to end the war quickly without costing the lives of more soldiers. So the president agreed to drop the bombs. It made the Japanese surrender because they knew that if they continued the war they could not win. The Japanese of the 1940's would have fought to the last inch of that island, and the body count would have been astronomically higher if the bombs weren't dropped (so this action actually also saved many Japanese soldiers lives). The atomic bomb's destructive power would have defeated them anyways, and this action successfully shortens the war. This action also gave the Japanese a serious blow.