Why did the United States drop two nuclear bombs on the Japanese mainland? It appeared that Japan was ready to surrender.
The official reason was to avoid a long and costly battle attempting to force the Japanese to surrender by invading the mainland. The Japanese were tenacious fighters and their tactics of Kamikaze suicide bombers and their courageous defense of their country in engagements such as the Battle of Okinawa, lend substantial credibility to this claim. Some such as General Eisenhower disagreed to as whether such a maneuver was really necessary.
Regardless of whether the Japanese government was seriously considering surrender, the bombing forced the decision and was less costly to the Americans (obviously) than a protracted war. Whether a protracted war would have spared Japanese lives as opposed to several more months of brutal warfare is an open question.
It is my opinion that this motivation was one of several competing reasons as to why the Americans decided to detonate nuclear bombs on Japanese civilian centers. In a large part I feel this was simply the natural evolution of the doctrine of total war applied to aerial bombings of civilian targets, first seen in action during the German bombing of Guernica and continued by the Americans both in the fire bombings of Dresden and Tokyo.
Beyond this there was also the ability to justifiably test an atomic bomb in warfare conditions. American military scientists were very interested in seeing the effects of nuclear weapons in many environments (even detonating them underwater to see what would happen). Obviously the most important environment to test it on would be that of urban or military targets. The prior would be unthinkable, outside of a war anyway.
Also, by deploying the bomb America was able to send a strong message about the balance of power after the war. Given that there is always some degree of uncertainty about political relations and military stability after the war, the bomb was certainly a strong message that Americans were not to be messed with. I don't think any of these motivations were alone sufficient in explaining why the American high command decided to drop the bombs; its far more likely that it was a combination of several of these and possibly additional concerns as well.
The Japanese had 4 terms they were demanding in order to "surrender":
To Westerners, this means that the Japanese would be "returning to barracks" and not surrendering in any way that word was understood to mean. The Allies stated clearly "unconditional surrender"
The Japanese felt that they could disgust and demoralize the Allies with the kamekaze attacks and drive the Allies to the bargaining table.
As for the 2nd atomic bomb, we knew what the Japanese were thinking because we were reading their codes. The high command claimed that since it took 4 years for the Allies to make the first atomic bomb, then it would take 4 more years for the second atomic bomb. They knew what an atomic bomb was, and the effects of such weapons, because they had 2 separate atom bomb projects themselves (one in Tokyo using chemical separation of Uranium isotopes, and a diffusion plant in what is now North Korea). Rhodes wrote 2 books (The Making of the Atomic Bomb and Dark Sun) that discussed this along with other national nuclear projects).
A conventinal land invasion of the "home islands" of Japan was expected to cost between 500,000 and 1,500,000 Allied troops. Based on how fierce the Okinawa fighting was, and that it took about 25 Japanese casualties to inflict 1 Allied casualty, it would be expected that a conventional land invasion of Kyushu and Honshu would end up having to kill tens of millions of Japanese civilians before they would really surrender.
Cook, in Japan at War, lists the number of soldiers in Japan at the time of surrender ate 4,335,500, with 3,527,000 stationed outside Japan (mostly in China and Korea).
In the end, the allies did not accept any of the conditions, but promised that the status of the emperor would be determined by the Japanese people.
The Soviet point of view was that the US used the bomb to threat the USSR.
According the Great Soviet Encyclopedy, article "Nuclear weapons" ("Ядерное оружие") "Применение ЯО не вызывалось военной необходимостью. Правящие круги США преследовали политические цели — продемонстрировать свою силу для устрашения свободолюбивых народов, запугать Советский Союз." "Use of the nuclear weapons was not justified by military necessity: the ruling class of the USA pursued political aims - to demonstrate their strength to the freedom-loving peoples, to threaten the USSR."
But my opinion is that the bomb was actually developed against Germany and only the fact that they surrendered so quickly saved them and brought such misfortune to Japan. The US already spent much money on the new weapon and just could not leave it unused.
It was also instrumental to demonstrate technical superiority over enemy (including Germany, which at the time was considered the most technologically advanced nation), so that to break an image of "advanced" and "civilized" Germans defeated by barbaric and underdeveloped inferior nations (the Germans did not consider the Anglo-Saxons sub-human, but still always empathized that the Germans are the most productive and creative). It was known that Germany developed a range of "wonder-weapons" so that their enemies had to get something to counter-balance such image.
There's a lot to explain.
Why was the bomb built?
This is too big to answer here! Read the great book The making of the atomic bomb. I'll quote it to answer the other questions.
Why the Allied policy of unconditional surrender?
When the Allied leaders met in 1943 at the Casablanca conference, the phrase 'unconditional surrender' was deliberately left out of the joint statement. But Roosevelt later used it in a hasty speech. and Churchill went along unquestioningly, rather than show any tension between the Allies.
Why did the Japanese refuse the Allied terms of surrender?
Why not invade Japan?
Why not ask Russia to help invade Japan?
Why drop atom bombs rather than more fire bombs?
Why not warn Japan about the bomb before dropping it?
Why not demonstrate the bomb on an unpopulated area?
Why was the bomb secret?
Why risk a nuclear arms race when the world sees the bomb?
Why not keep the bomb secret from Russia?
Byrnes' most dangerous misunderstanding from Szilard's point of view was his reading of the Soviet Union: >
Although this question has been well answered, I cannot help but remark that its basis seems quite shaky, and perhaps somewhat pejorative: Seeking to portray the USA as a "bully" in its use of the Bomb.
Even if we grant "they were already on the winning side" (a point that the questioner has failed to document), this was not a valid reason to decline from using the Bomb. The fact remains that on August 5, 1945, the USA had not yet won the war, but was still heavily engaged in the Pacific theatre.
When would the USA actually achieve the victory that they were 'already' heading towards, were they to opt for a victory using only conventional methods of warfare? Would it be six months? A year? This was not clearly known. What would the cost of such a victory be? How many millions of dollars? How many hundreds of thousands of lives, both American and Japanese, would it take? Again: Not at all clear.
[ As an aside I might add that warfare is never entirely predictable. Even the best generals and the most sophisticated technology do not assure victory: Both History (the American Revolution for example) and the world of 2013 (Afghanistan for one) clearly confirm that. Victory in war is never assured until it is achieved. ]
Regardless of the answers to these questions concerning a conventional victory, one thing seemed quite certain: Using the A Bomb would greatly expedite an American victory and minimize its cost, both in blood and in treasure - as indeed History confirms.
So, "they were already on the winning side" was by no means a valid reason for declining to use the Bomb. "Already on the winning side", i.e. the question of an American victory, was not issue. The issue was the cost of such victory, and using the Bomb was clearly the USA's best option in terms of that "cost/benefit analysis".
By way of analogy: Two individual are engaged in hand to hand combat, to the death. They are pummeling one another with severely damaging blows. One of the combatants also has a pistol available to finish the fight quickly, could he only endure his opponent's batterings and get his hands on that pistol.
Now: Even if the armed fighter know he would be able to prevail and pummel his opponent to death, would he not seek to use the pistol rather than endure the many blows and potential injuries that hand to hand combat would require him to endure? In the same way, the USA used the Bomb against Japan.
The war in Europe had brought the powerful red army, which was mostly responsible for beating Nazis to Berlin in Germany. Us and the soviet Union were having some disagreements on arragements on Europe. Us wanted to let the russians know who is the boss (in the words of Truman) and thus US army roasted nearly half a million Japanese and hurt millions in the coming generations to achieve that. President Truman was instrumental in this by failing to stop the hawks in the army. FDR (Rosevelt would never allow the vision of post war power grab to extend to nuking Japan.) Japanese cities were already incinerated and the most worrisome thing for the Japanese was a Russian Invasion and not the nuclear attack. Also they could not stomach the hanging of their emperor. US delayed assurances to safeguard the emperor until it had exploded the second bomb, on the same day USSR attacked Manchuria, then controled by Japanese forces. Japanese were considered to be sub human at that time and thus the political cost for US politicians was negligable. To understand this well watch Oliver Stone's documentary "Untold history of the USA" episode 2 and 3.
Compare to German, Japan have controlled 1/6 part of world on august 1945.
Japanese are kamikaze.
Hiroshima was a main military port and main headquarters.
Nagasaki was a city-military-factory, Mitsubishi produced motors of aircrafts.
It was like nuking The Pentagon and Lockheed Martin's main factories.