Some other possibilities, as the length of time between the successful testing of the bomb and our actually dropping it was a mere three weeks, and the decision-making process to do so is a little muddled historically.
First, the Manhattan Project was designing a bomb to be dropped on Germany as soon as we developed it. However, we were not able to test one until July 16, 1945, and Germany surrendered on May 7th. The decision to focus on Japan then was a quick one, and with a new President, Harry Truman.
Hiroshima and Nagasaki were not terribly important militarily, but they were two of the only cities not yet bombed, and still mostly intact. We wanted to see the full effect of the atomic bomb on an undamaged city. This lends strength to the argument the bomb was dropped to scare the Soviets as well as to convince Japan to surrender.
Another reason, and I think the most compelling, was out of hatred and revenge. Revenge for the Bataan Death March, for Pearl Harbor, for how our prisoners were treated, for the Kamikaze attacks, etc. There was simply no way our government or military were capable of deciding anything other than using the weapon once we had it.
The bomb was actually dropped mainly to demonstrate to the Soviet Union that we had it. Truman believed very strongly that he had to do something to show Stalin who was boss and this was one way they thought they could do it. The idea of ending the war was one that was emphasized after the fact to justfiy the horror they'd created . The casualty estimates for an invasion of the home islands was raised dramatically in the five to ten years after the bombs were dropped, and even people high up in the government in the US and elsewhere knew that the Japanese were in the process of suing for peace but it would take several more weeks. The invasion of Japan was unlikely at the very outside but again was emphasized after the fact, particularly once we found out that the Russians had already gotten most of the technology and would have a bomb within a few years.
Of course the above posters also have good answers and this should show the fact that history is all about interpretation, opinion, and speculation and not simply a matter of facts and figures as we often think it is.
The dropping of the atomic bombs was motivated for a variety of reasons. One reason that was most evident was to end the conflict with the Japanese. The belief was that there was a need to stop the death of U.S. soldiers engaged in battle with the Japanese and the dropping of the bomb was seen as a way to end the conflict, and the death of military personnel, in a quick manner. A more subterranean reason could be considered when evaluating the relationship between the United States and the Soviet Union. With the defeat of Hitler, the shared enemy of both nations gone, the distrust between them began to emerge. Some in the United States believed that the display of the atomic capacity would scare the Soviets into immediate submission, giving the United States singular superpower status.
Hiroshima was bombed on August 6, 1945 and Nagasaki was bombed on August 9th. These are the only two atomic attacks that have occurred in history.
By dropping the atomic bomb on Hiroshima and Nagasaki we showed the whole world how technologically advanced we were. We had capabilities that other nations did not have. After we dropped these bombs, World War II ended very quickly. On August 16th, Japan surrended to the allied powers.
The atomic bombings led Japan to adopt the "Three Non-Nuclear Principles." Although it was never adopted into law it stated that Japan will never have nuclear weapons in their country. They came to this conclusion because of the devastation they seen caused by the bombs droped in their country at the end of WWII.
The US dropped the atomic bombs on Hiroshima and Nagasaki in order to end the war more quickly and without so much bloodshed. The fighting, as the US got closer to Japan was so bloody that it made American planners believe that something like one million American troops would be killed or wounded in an invasion of Japan.
Some historians believe that US policy makers also decided to drop the bombs so as to end the war before the USSR could take part of Japan and hold on to it the way they took part of Korea and made it into what is now North Korea.
USA was fighting a major bloody war with Japan when they dropped a bomb on Japan. This war well known as World War II had caused major devastation for people of countries on both sides of the war. Any action at that time taken to end the war swiftly was definitely justified to some degree.
USA dropped atom bomb on Japan to achieve just that objective, which it did achieve in reality also. Whether this option used by USA was the best is open to discussion. I personally believe, it may have been worthwhile to demonstrate the power of atom bomb without causing so much destruction, and then used this demonstration to force Japan to surrender. But his may or may not have achieved the desired result. In case of failure, it could have further strengthened the Japanese resolve to fight and thus cause more destruction.
Basically it was to end the world word 2.
Japan was not surrendering despite the world's oppression, so the us president felt that japan needs something strong to stop them.
Righ after the drop of atomic bomb in Nagasaki and Hiroshima, the Japan surrendered and the war ended.
Apart from that, some experts think that the drop of atomic bomb also somewhat frightened soviet forces which was a benefit for US during the cold war.
But other experts say that there was no such ulterior motive, and US dropped it because they have spent millions of dollars to build the bomb and they could not waste it.
it was kind of out of topic but basically, the bomb was dropped in order to end the war quickly but killed so many people in return.
Perhaps one of the reasons that motivated the Americans to drop the atomic bombs on Japan was the fact that by using the bombs, the US could seek to control the circumstances of post-war Japan and to impose American control in the redesigning of Asia. The sheer costs and effort spent on developing the nuclear warhead during the Manhattan Project also meant that the concentration of cutting-edge scientists and engineers employed were generating a momentum that could not be stopped - they wanted to see the atomic bombs in use and Japan provided the best opportunity, at that time, to do so. Casualties were also a key factor. Truman had been informed by his military advisers that it was necessary to invade the Japanese mainland since the imperial army would fight on a suicidal scale to counter the American troops. Due to such a tenacity in fighting, a high casualty count was to be expected from the American forces. This estimation greatly influenced Truman in his decision-making, since the operationally-ready nuclear weapons could be used to force Japan to surrender without an invasion of the mainland and the incursion of high casualty numbers - they could be used to preserve American life.
Certainly it saved many American Lives. But in addition, because of the no-surrender culture of Imperial Japan, had a mainland invasion succeeded, there likely would have been more Japanese killed on the home islands from conventional warfare.
Germany was militarily defeated long before its' surrender. But Hitler insisted on fighting until the Russians were within hundreds of meters of the his headquarters in Berlin. It still took until May 7 (or 8th depending on your version) before Nazi Germany surrendered.
And the new Reich President of Germany, after Hitler's suicide, Admiral Doenitz, didn't surrender his government until about May 23, 1945.
Imagine having to occupy all of the Japanese home islands and what the battle of Tokyo would have been like! (For both sides).