I want to express an appreciation to everyone on this thread because we need to be concerned with this issue in the context of our place in history and the potential for terrorists to bring it to the forefront.

The history of the development of the Atomic Bomb is one of the most interesting parts of the Twentieth Century and represents a level of effort and achievement among the greatest in human history. For those of you with an intrinsic interest in the story I recommend the following:

Richard Rhodes: Making of the Atomic Bomb
Richard Rhodes: Dark Sun
Stephane Groueff: The Manhattan Project
Leslie Groves: Now It Can Be Told
Movie: The Day After Trinity

Now for MY GRAND SIMPLIFICATION: I'd say the motivation on the part of the scientists and engineers was twofold:
1) Intrinsic interest in the basic laws of physics being understood and turned to application.
2) Concern that the enemy, particularly Germany, would be working on the weapon concurrently and possibly be ahead of the Allies.

Items I consider facts:
The Allies, primarily Great Britain and the United States spearheaded the effort with the aid of immigrant scientists from Italy, Russia, Germany, Denmark. Only the United States had the money and raw industrial power to actually isolate Uranium 235 using two entirely separate methods of concentration. An entirely different project used the first nuclear reactors to produce plutonium.

An operation by the Allies called Operation Paperclip established as the Allies re-conquered Europe established that the German efforts to produce nuclear weapons were considerably behind the Allies. But by then the Manhattan Project was at high speed and project inertia and the fact that the Japanese were still very much in the picture led to no diminution in the Allied effort.

Despite major and costly emphasis on secrecy and project security, early on the Soviets were the beneficiaries of information from the heart of the project due to Communist penetration at every phase and part of the Manhattan Project. They even got samples of bomb-quality Uranium.

Riding like a monkey on the back of the theoretical researchers was the knowledge that as horrible as the atomic bomb appeared to be, should it be possible, since 1942 the idea of using the energy released from the fusion of hydrogen into helium showed that in theory, an energy source much much greater than that of the atomic bomb might be possible. They called this the 'Super' and tabled it while they developed the atomic device.

There was so little doubt of the success of a Uranium bomb that it was never tested. This was Little Boy, destined for Hiroshima. The device that was tested at Alamogordo (Trinity) was a Plutonium device, and the means of detonation were sophisticated enough to require their own section of Los Alamos and their own test procedures. The Nagasaki weapon was a plutonium device called Fat Man.

At the time the bombs were dropped on Japan, there was material available and staged for a third weapon. The political leaders of the United States at that time ordered the military to return the makings of that weapon to the US Mainland. Until that time, the military had physical control of all nuclear weapons and could have had the third one used, although there is no evidence that it was ever contemplated.

While the atomic bomb was being researched and developed, an aviation program was developing an extremely expensive long range bomber, the B-29, a project comparable in expense to the Manhattan Project. A bomber group of B-29s was created in order to train for atomic weapons delivery. Already at that time, almost every Japanese city that could be called that was being bombed to smithereens. Thousand Plane raids were conducted against Tokyo that killed more people than died at Hiroshima. Aware of the possibility of using a new weapon, several cities in Japan were 'saved' from further conventional attack so the efficacy of the new weapons could be determined.

I don't think there is much to argue with in the above. It's when we start to put value judgements on these and succeeding events that the excitement starts. There is a strong urge to play historical 'what ifs', and I've seen a lot of plain inaccuracies in this thread.

Records indicate that the use of atomic weapons was keenly discussed in Los Alamos and in The White House. The prospect of a 'warning' with or without demonstration was considered. When Kyoto was proposed as a target, Americans well aware of the cultural significance of the city had it spared.

Rather than insist on some kind of moral ineptitude or uprightness, I think it is clear that the US government went into action after thought and reflection. At the time we were engaged in a total war with an enemy who had displayed great persistance, ingenuity, and effectiveness.

I don't think it is necessary to go into whether or not we cared what the Soviet Union thought of our weapon. They were well aware of it and working hard to develop their own at a far greater cost to their own people.

The Japanese people were long suffering slaves of their own government, which was led by a god, the emperor Hirohito. The decision to give up was not one they were capable of making.

So, my answer to the title of this thread:

The US dropped THE BOMB to defeat the Empire of Japan and end World War II. In this it proved to be a success.

To me, the lesson we must learn is:

The weapons that were used on Hiroshima and Nagasaki were ONLY atomic weapons. They destroyed whole cities on a scale already achieved by thousand plane raids. When the first hydrogen device was detonated in November 1952 it was of a magnitude unimaginable just seven years before. The island on which it was placed ceased to exist.

Another lesson is that once a country establishes a nuclear infrastructure in order to produce one bomb, more bombs will follow. You don't spend billions of dollars to concentrate bomb grade Uranium or produce Plutonium and just stop at one bomb. You have a de-facto production line going. So rather than dwell on whether or not certain decisions were moral, or right or wrong, I think we should work together to determine whether we want North Korea, Iran, and terrorists to pick up where Pakistan has left off thanks to the wholesale leakage of nuclear information and materials by Dr. Abdul Qadeer Khan.