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.
"...in 1945... Secretary of War Stimson, visiting my headquarters in Germany, informed me that our government was preparing to drop an atomic bomb on Japan. I was one of those who felt that there were a number of cogent reasons to question the wisdom of such an act. ...the Secretary, upon giving me the news of the successful bomb test in New Mexico, and of the plan for using it, asked for my reaction, apparently expecting a vigorous assent.
"During his recitation of the relevant facts, I had been conscious of a feeling of depression and so I voiced to him my grave misgivings, first on the basis of my belief that Japan was already defeated and that dropping the bomb was completely unnecessary, and secondly because I thought that our country should avoid shocking world opinion by the use of a weapon whose employment was, I thought, no longer mandatory as a measure to save American lives. It was my belief that Japan was, at that very moment, seeking some way to surrender with a minimum loss of 'face'. The Secretary was deeply perturbed by my attitude..."
Dwight Eisenhower, Mandate For Change, pg. 380
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":
The emperor would remain inviolate.
Japan's borders would be restored to those of summer of 1942, requiring the allies to return to Japanese control every island and country that they had been thrown off of, such as Guadalcanal, Iwo Jima, and the Philippines.
Japanese troops would surrender only to Japanese officers. No allied officer or soldier would be allowed to be involved. In effect, the Japanese troops would return to barracks.
The Japanese would pay no reparations to anyone for what they did.
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 kamikaze attacks and drive the Allies to the bargaining table.
As for the 2nd atomic bomb, the United States knew what the Japanese were thinking because it was 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 conventional 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 threaten the USSR.
According the Great Soviet Encyclopedia, 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."
"The use of nuclear weapons was not justified by military necessity: the ruling class of the USA pursued political aims - to demonstrate their strength to 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.
In January 1943, Franklin Roosevelt met with Winston Churchill at Casablanca. In the course of the meeting the two leaders discussed what terms of surrender they would eventually insist upon; the word "unconditional" was discussed but not included in the official joint statement to be read at the final press conference. Then, on January 24, to Churchill's surprise, Roosevelt inserted the word ad lib: "Peace can come to the world," the President read out to the assembled journalists and newsreel cameras, "only by the total elimination of German and Japanese war power.... The elimination of German, Japanese and Italian war power means the uncon- ditional surrender of Germany, Italy, and Japan." Roosevelt later told Harry Hopkins that the surprising and fateful insertion was a consequence of the confusion attending his effort to convince French General Henri Girard to sit down with Free French leader Charles de Gaulle:
We had so much trouble getting those two French generals together that I thought to myself that this was as difficult as arranging the meeting of Grant and Lee-and then suddenly the Press Conference was on, and Winston and I had had no time to prepare for it, and the thought popped into my mind that they had called Grant "Old Unconditional Surrender," and the next thing I knew I had said it.
Churchill immediately concurred- "Any divergence between us, even by omission, would on such an occasion and at such a time have been damag- ing or even dangerous to our war effort" -and unconditional surrender became official Allied policy.
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?
To Szilard's argument that using the atomic bomb, even testing the atomic bomb, would be unwise because it would disclose that the weapon existed, Byrnes took a turn at teaching the physicist a lesson in domestic politics:
He said we had spent two billion dollars on developing the bomb, and Con- gress would want to know what we had got for the money spent. He said, "How would you get Congress to appropriate money for atomic energy re- search if you do not show results for the money which has been spent al- ready?"
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: >
Byrnes thought that the war would be over in about six months.... He was concerned about Russia's postwar behavior. Russian troops had moved into Hungary and Rumania, and Byrnes thought it would be very difficult to per- suade Russia to withdraw her troops from these countries, that Russia might be more manageable if impressed by American military might, and that a demonstration of the bomb might impress Russia. I shared Byrnes' concern about Russia's throwing around her weight in the postwar period, but I was completely flabbergasted by the assumption that rattling the bomb might make Russia more manageable.
The bombing of Japan was a warning to the USSR. The allies knew that Japan would surrender at the drop of a hat without a fight as the had actually asked to be allowed to surrender ten times before the first bomb was dropped.
The official reason for denying them a surrender was that they had placed various requirements on the surrender but in actual fact almost all of these demands were actually accepted in the end - the biggest request from the Japanese PoV being, of course, the continuance of the Emperor's position which was granted.
There was never any chance of a fighting invasion of the Japanese mainland, and everyone knew that well in advance. That idea is simply propaganda. The Japanese were beaten and they knew it. If there was the odd general who wanted to fight on the simple fact was that he would have had to do it himself as the army was on the verge of mutiny, as was the remains of the air force. Again, the vision of the unconquerable samurai who would die before surrendering is a comfortable myth bolstered by a handful of freaks like Hiroo Onoda. Such imagery is no more a true picture of the Japanese army than the Alamo is of the US army en mass.
When MacArthur presented Japan's documentation on the subject of being allowed to surrender, Truman reportedly dismissed the idea without even reading the proposal, commenting that MacArthur was a great general but a lousy politician - a strong clue that the bombing was a political event rather than a military one.
The reason for the second bomb has been debated but is likely to be a combination of two main reasons: firstly to test the second device's design which was substantially different from the first; secondly to hint to Stalin that the US had a supply of these things, not just one made through some super-human effort which would be hard to replicate quickly.
It is easy to forget the degree to which the Japanese had been dehumanised in the US. The idea that a bomb design be tested by being dropped on civilians would not have raised anything like enough of a protest in the US if it had been floated beforehand and, indeed, continued to be regarded as completely justified for decades afterwards in general public opinion.
The ultimate roots of the bombing are a fascinating story of the interaction of militarism and religion on both sides, going back to the days (less than a century before) of Admiral Perry's expedition to "open" (ie, threaten to bombard into submission) Japan and the reaction of the Shogunate to that challenge. In light of that, there is a horrible irony in the fact that Nagasaki was bombed (due to weather), as it was one of the first cities opened to the outside world and was opened specifically as an attempt to avoid Japan being attacked and conquered by the US.
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.
... on a Weakened Japan
Compare to German, Japan have controlled 1/6 part of world on august 1945.
Japanese are kamikaze.
During the Meiji period, Nagasaki became a center of heavy industry. Its main industry was ship-building, with the dockyards under control of Mitsubishi Heavy Industries becoming one of the prime contractors for the Imperial Japanese Navy, and with Nagasaki harbor used as an anchorage under the control of nearby Sasebo Naval District. These connections with the military made Nagasaki a major target for bombing by the Allies in World War II.
During World War II, the 2nd General Army and Chugoku Regional Army were headquartered in Hiroshima, and the Army Marine Headquarters was located at Ujina port. The city also had large depots of military supplies, and was a key center for shipping.
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.