Pros and Cons
Was the US justified in dropping the atomic bombs on Japan?
Why the bomb was needed or justified:
The Japanese had demonstrated near-fanatical resistance, fighting to almost the last man on Pacific islands, committing mass suicide on Saipan and unleashing kamikaze attacks at Okinawa. Fire bombing had killed 100,000 in Tokyo with no discernible political effect. Only the atomic bomb could jolt Japan's leadership to surrender.
With only two bombs ready (and a third on the way by late August 1945) it was too risky to "waste" one in a demonstration over an unpopulated area.
An invasion of Japan would have caused casualties on both sides that could easily have exceeded the toll at Hiroshima and Nagasaki.
The two targeted cities would have been firebombed anyway.
Immediate use of the bomb convinced the world of its horror and prevented future use when nuclear stockpiles were far larger.
The bomb's use impressed the Soviet Union and halted the war quickly enough that the USSR did not demand joint occupation of Japan.
Additional Notes on why the bomb was needed or justified:
Japan was ready to call it quits anyway. More than 60 of its cities had been destroyed by conventional bombing, the home islands were being blockaded by the American Navy, and the Soviet Union entered the war by attacking Japanese troops in Manchuria.
American refusal to modify its "unconditional surrender" demand to allow the Japanese to keep their emperor needlessly prolonged Japan's resistance.
A demonstration explosion over Tokyo harbor would have convinced Japan's leaders to quit without killing many people.
Even if Hiroshima was necessary, the U.S. did not give enough time for word to filter out of its devastation before bombing Nagasaki.
The bomb was used partly to justify the $2 billion spent on its development.
The two cities were of limited military value. Civilians outnumbered troops in Hiroshima five or six to one.
Japanese lives were sacrificed simply for power politics between the U.S. and the Soviet Union.
Conventional firebombing would have caused as much significant damage without making the U.S. the first nation to use nuclear weapons.
Additional Notes on why the bomb was NOT needed or unjustified:
The Decision to Use the Bomb
The modern nuclear arsenals and the struggle to control nuclear weaponry have brought new significance and controversy to the American use of the atomic bomb in World War II. This reading selection describes the circumstances surrounding the decision to use the atomic bomb. There is considerable debate among historians about the necessity of using the bomb to force Japan's surrender; there is perhaps even greater controversy concerning the moral principle involved in subjecting the two Japanese cities of Hiroshima and Nagasaki to this weapon. This latter point is raised, but not answered, at the end of the essay.
World War II was the second world-wide war in less than a generation's time. The World War I had erased any romantic illusions about the nature of modern war; World War II saw the complete mobilization of entire populations and economies in the waging of the war. It was fought with grim determination on every side. In such conditions, each side carried out acts of great brutality in the frustration and necessity of achieving victory.
For the first time outside a civil war, fighting spread beyond the armies to whole populations: Hitler used aerial bombing to try to break the spirit of the British; the Japanese used aerial bombing and soldiers against the Chinese civilian population; both Japan and Germany used their military forces to subdue resistance in occupied nations; and the allied forces used bombing to carry the war beyond the battle front and break the opposition of enemy populations. By the end of the war, technology had advanced to the point where such bombings were terrible: the allied bombing of Dresden killed tens of thousands of people, and the American firebombing of Tôkyô in March 1945 probably killed more than 100,000 people.
During this period, wartime technology raced ahead, as each side attempted to be the first to develop the techniques and equipment that would enable it to win. Many nations sought to decipher the secrets of atomic energy, but the United States was the first to develop the ultimate weapon, the atomic bomb.
Prelude to the Bomb: On April 1, 1945, the Allies invaded the southern Japanese island of Okinawa, and their victory there after bitter and bloody fighting with heavy losses on both sides proved that Japan could not win the war. It also proved, however, that invasion of the Japanese homeland would cause massive casualties on both sides. As American ground forces swept Okinawa clean of Japanese troops, the local civilians were caught in the middle. Subjected to gun fire, bombing, and infantry combat by the American advance, they were prevented from surrendering by the Japanese troops. Okinawa only served to confirm everyone's idea of how the final battle for the main islands of Japan would be fought.
The surrender of Okinawa caused the Japanese cabinet to collapse and a new, pro-peace prime minister and foreign minister pressed the army to allow negotiations. The Japanese military, however, trapped in its own mystique of rigid determination and self-sacrifice in the name of the nation and emperor, insisted on strict terms.
Just at this point, the atomic bomb became a reality. The first successful test of the atomic weapon was held on July l6, 1945. The United States now had the choice of using it to try to end the war in another way. All other forms of attack, from the grim battle for Okinawa to the terrible fire bombing of Japan's cities, had failed to deter the leaders in Tôkyô. Perhaps the atomic bomb would resolve the crisis without a need for invasion. President Truman, who had already left for Potsdam to meet with Churchill and Stalin, left instructions that the bomb was not to be used against Japan until after the Allies had agreed on and issued a declaration.
The Potsdam Declaration of July 26, issued by the Allied powers and calling for "unconditional surrender," was not acceptable to the Japanese military, despite the declaration's threat that failure to surrender would be met by "complete destruction" of the military and the "utter devastation of the Japanese home land." Following ten days of Japanese silence, the atomic bomb was dropped on August 6, 1945, on the city of Hiroshima.
The Impact on Japan: It was reported the next day to the Japanese Army General Staff that "the whole city of Hiroshima was destroyed instantly by a single bomb." On August 8 the army was further rocked by the news that the Russians, who had remained neutral to Japan throughout the war, had attacked Japanese forces on the Asian mainland. But despite the prime minister's insistence that Japan must accept surrender, the army insisted on total, last-ditch resistance. The news, midway through this conference, that the city of Nagasaki had also been destroyed by another atomic bomb, did not sway them from their determination.
Finally, the Japanese prime minister and his allies agreed that the only course was to have the emperor break the deadlock by expressing his view. The emperor's statement that Japan's suffering was unbearable to him and that he wished for surrender broke the military's opposition and began the process of ending the war in the Pacific.
Assessing the Decision: Was it necessary to use the atomic bomb to force Japan to surrender? This is a subject of heated debate among historians. Some point to the existence of a pro-peace faction in Japan, resisting the army and growing in strength. This faction had already tried to express Japan's interest in peace through the Russians, whom they believed were still neutral. In fact, the Russians had secretly agreed at the Yalta Conference in February 1945 to attack the Japanese.
Moreover, Japanese offensive capabilities were exhausted. The navy and air force were almost totally destroyed by the summer of 1945, and the Japanese islands were completely cut off from the rest of the world. The Russian attack of August 8 on Manchuria met little or no resistance.
More credible sources with a lot of information: http://www.colorado.edu/AmStudies/lewis/2010/atomic.htm#Critical