The decision to drop the atomic bomb in August 1945 was taken to avoid further Allied casualties in the Pacific War by forcing Japan into an immediate surrender. Another reason was, as the victors jostled for control of the broken empires of the losers of the Second World War (1939 to 45), to demonstrate to the USSR that in the post-war world, the US had the ultimate weapon of destruction.

Hiroshima and Nagasaki were selected as targets because they were large and relatively undamaged cities, and so when they were destroyed by a nuclear attack, the impact would be all the more shocking on the hardline Japanese military government that seemed determined to fight on at any cost. The dropping of the bombs did significantly contribute to Japan's surrender, which came shortly after in mid-August.

The Manhattan Project, the secret code name for the United States' research programme into developing an atomic bomb, began in earnest in August 1942. By the spring of 1945, the scientific team, led by nuclear physicist Robert Oppenheimer (1904 to 1967), had achieved its objective. The project had involved over 100,000 individuals and racked up costs of around $2.5 billion. The 'Trinity' test at a remote spot in the desert near Alamogordo, New Mexico, on 16 July 1945, proved that this was a devastating new weapon that could wipe out cities in a single blow. The explosion, equal to 18,600 tons of TNT, had destroyed all plant and animal life within a mile radius. The shockwave was so powerful that it blew out windows 200 miles (322 km) away. The blast had been ten times more powerful than the scientists had expected.

Not just the war but the fate of the entire world had changed overnight. Oppenheimer was reminded of the line from the sacred Hindu text the Bhagavad Gita: "Now I am become Death, the destroyer of worlds" (Marston, 292).

The Manhattan Project had been conducted for several reasons. The first was that, with such a weapon, the US could win the war. The second was to ensure US weaponry could match anything secretly being developed in Nazi Germany and the USSR (as it happened, neither state developed an operational atomic bomb by 1945). Another reason, one that became clearer in the latter stages of the conflict, was that possession of such a terrible new weapon would mean the US could dominate the post-war world.

As Germany's defeat looked inevitable by early 1945, both the US and USSR began to negotiate with each other for who would control the territories once (and some still in 1945) occupied by the Axis powers of Germany, Italy, and Japan. This rivalry took the form of diplomatic discussions and bargaining at such meetings of Allied leaders as the Tehran Conference (Nov-Dec 1943), Yalta Conference (Feb 1945), and Potsdam Conference (July-August 1945). The demands made at these conferences were much more likely to be achieved if the claimant was actually in possession of the territory in question. Joseph Stalin (1878 to 1953), the leader of the USSR, was determined, for example, to keep Poland as a Soviet satellite state. US President Franklin D. Roosevelt (1882 to 1945) and British Prime Minister Winston Churchill (1874 to 1965) were very keen to keep Greece as a free and independent democracy. These are just two examples of numerous parts of the global map which were disputed in this game of post-war empires.

The US's and Britain's relations with the USSR were ambiguous. In 1944, the Allies wanted the USSR to join the war against Japan. In return for this, certain Soviet demands would be met. These demands included control of the Kuril (aka Kurile) Islands, the southern part of Sakhalin Island (both of these are in the Sea of Okhotsk and had been occupied by Japan), and retaining the status quo regarding Mongolia, which had been a Russian client state since 1924. By 1945, however, the US and Britain were concerned that if the USSR did enter the war against Japan, Stalin might then make territorial claims in East Asia. As it happened, the USSR did not declare war on Japan until 9 August 1945 (after the atomic bomb had been dropped).

The Potsdam Conference delivered an ultimatum to Japan in the form of the Potsdam Declaration, made on 27 July 1945. Japan was to make an unconditional surrender or face "prompt and utter destruction" (quoted in Smurthwaite, 133). The Japanese military government ignored the declaration, perhaps believing there was still some room for negotiation. There was, however, following Japan's rejection of the ultimatum, now no doubt in the Allied leaders' minds (if indeed there ever had been) that the atomic bomb would be used. This was true of both Churchill and Roosevelt's successor, Harry S. Truman (1884 to 1972), and the top military commanders. The question was not if, but rather, when and where?

Aware of the terrible destruction an atomic bomb could inflict on the enemy after the 'Trinity' test, Allied leaders realised a clear justification was needed to convince the public that such drastic measures were necessary. This public justification masked another motivation, but it was nevertheless still widely considered to be true by all involved. The idea was that only the dropping of atomic weapons would bring a Japanese surrender and avoid the terribly high casualties on both sides that a conventional amphibious landing on Japan's main islands would inevitably entail.

So far, all through the Pacific War, US and Allied forces had suffered dreadful casualties as they took various Pacific island archipelagos which Japanese forces had occupied. At the Battle of Okinawa in April, May, and June 1945 – what turned out to be WWII's last major battle – around 100,000 Japanese combatants, up to 150,000 civilians, and around 12,000 US troops were killed.

The majority of Japanese soldiers fought to the death in the Pacific War since they had been indoctrinated to believe that surrender was dishonourable and that there was nothing more glorious than giving their own lives to secure the life of the emperor and Japanese soil from foreign invasion. Kamikaze suicide missions had been employed throughout 1945, and more were being prepared should an invasion come. The heavy Allied bombing of Japanese cities combined with a naval blockade that deprived the military of the materials to wage war had brought Japan's military to its knees. Many commanders in the US Navy claimed they had already won the war. In contrast, commanders in the US Air Force claimed it was they who were about to win the war if they could continue the bombing campaign a little longer. One hard fact remained, though: Japan had still not surrendered.

There remained formidable defences across Japan, as here summarised by the historian D. Wright:

Japan still had 2,350,000 regular troops, 250,000 garrison troops, 7,000 aircraft, 4,000 employees of the armed services and 23,000,000 men, women, boys and girls sworn to fight to the death. Adding the kamikazes and the remnants of the navy provided the ingredients for a bloodbath that would make previous battles pale into insignificance.
(76)

Even if an invasion were successful, and there was no guarantee it would be, Allied experts and planners for an invasion, code-named Operation Downfall, predicted at least a 35% casualty rate amongst the US troops involved, just as there had been at Okinawa. The battle for Japan would prolong the war into 1946 or even beyond. The Pentagon estimated at least half a million US dead or wounded from such an operation. The Purple Heart medal is given to wounded-in-action US service personnel, and a huge stockpile of them was accumulated (in fact, enough were made in 1945 to last right through the Korean War, the Vietnam War, and well into the 21st century). There was also the consideration that over 100,000 Allied prisoners of war were being held in Japan, and it was feared these could be executed should an invasion occur.

As Winston Churchill noted:

At any rate, there never was a moment's discussion as to whether the atomic bomb should be used or not. To avert a vast, indefinite butchery, to bring the war to an end, to give peace to the world, to lay healing hands upon its tortured peoples by a manifestation of overwhelming power at the cost of a few explosions, seemed after all our toils and perils, a miracle of deliverance.
(Moskin, 295)

The Allies had been fighting for over five years across the globe. The risk to yet more Allied soldiers was very real, and so the attraction of two aircraft dropping just two bombs to end the war was seemingly irresistible. This was the message for the public, but it was also the private opinion of many of the leaders involved. Anthony Eden, then British Foreign Secretary, later stated:

I think we had no choice but to use it …how terrible the alternative must have been and how long it must have gone on and how much suffering it would have created…I don't think that anybody could have voluntarily foregone the possibility of shortening the war, despite the horror of the weapon.
(Holmes, 571)

There was a second, more subtle reason for dropping the bomb: to demonstrate to the USSR that the US now had a weapon to support the US vision of the post-war world. Indeed, this motivation had been there all along while billions of dollars were invested in the Manhattan Project. As the project's overall supervisor, General Leslie R. Groves, noted:

There was never from about two weeks from the time I took charge of this project any illusion on my part but that Russia was our enemy and the project was conducted on that basis.
(Dear, 57)

Certainly, President Truman felt that "only an actual atomic raid would convince Stalin of the irresistible force behind America's diplomacy" (quoted in Liddell Hart, 438). This reinforcement was deemed more necessary than ever following what was felt to be Stalin's reluctance to adhere to agreements made at conferences like Tehran and Yalta. Truman was fully aware that many people in the US were of the opinion that the USSR was getting the better deal concerning who controlled what in the post-war world, a perception evidenced in the common saying that the US had 'won' the war but 'lost' the peace.

The question remained of where exactly to strike. Two targets were needed because the Manhattan Project had developed two different types of bomb. There was one using radioactive uranium and another using plutonium to kick off a nuclear reaction. Two attacks might also convince the Japanese government that more would follow, even if the US did not actually have more nuclear bombs at the time. If two major cities were hit in quick succession, then Japan's hardline military government might well be shocked into an immediate surrender.

The US Air Force Target Committee chose its two targets for an atomic bomb drop for very particular reasons. The scientists required urban targets bigger than the anticipated explosion to be able to assess the damage potential. It was no good wiping out a small town when most of the destructive nuclear energy would be 'wasted' on the surrounding countryside. A second requirement was to have a target not too badly damaged already by conventional bombing, again in order to more accurately assess the destructive power of the nuclear device. In addition, the more pristine the atomic bomb targets were, the greater the psychological impact of the attack would be. For the very same reason, it was decided not to give any warning prior to the attack when it did finally come. Finally, the targets had to possess military and industrial installations so that the attack was not made directly, at least officially, against civilians.

The bombing of Tokyo in 1945 using incendiary bombs had caused so much destruction that the capital was not a viable option for an atomic attack. Another 63 cities were dismissed for the same reason. With some difficulty, a list of 10 potential targets was drawn up. Two cities that were large enough and still relatively unscathed by the US bombing campaign were placed at number one and three on the list: Hiroshima and Kokura (now part of Kitakyushu). Number two on the list, Kyoto, was rejected because of its historical and cultural importance; the consideration here being that its destruction might spur the Japanese people to fight on.

Hiroshima, located on the delta of the Ota River, met all the requirements. In addition to a large industrial centre, it was an important army assembly point. Hiroshima had a population of 280,000 civilians and 43,000 soldiers. Another advantage of Hiroshima was that it did not have a prisoner-of-war camp. The uranium bomb 'Little Boy' was dropped by the Boeing B-29 Superfortress Enola Gay on the city of Hiroshima on 6 August 1945.

A plutonium bomb variant, given the name 'Fat Man', was dropped by the B-29 Bockscar three days later, but not on Kokura as planned because thick clouds had covered the target area. Nagasaki, which had also been on the 'top 10' list, was selected as an alternative target. It was cloudy over Nagasaki, too, and the bomb was dropped 1.5 miles (2.4 km) from its planned aiming point. Consequently, the effects of the bomb were not quite as powerful as predicted, and the Mitsubishi dockyards and torpedo factories were not, as had been intended, totally destroyed.

Estimates vary, but perhaps as many as 140,000 people were killed in the attack on Hiroshima and 74,000 people in Nagasaki. In both atomic bomb attacks, there were tens of thousands of injured survivors, and even more who later suffered the negative effects of radiation exposure. Perhaps the real death toll was 300,000 people. As Tokyo Radio announced: "The dead are too numerous to be counted. The destructive power of these bombs is indescribable" (Moskin, 303).

The two bombs achieved their primary practical purpose of shock and awe. In addition, the USSR's Red Army immediately invaded Japanese-occupied Manchuria. Stalin, keen to stake a claim for East Asian territories, had ignored the secret diplomatic overtures from Japan seeking peace. With the threat of more atomic horror and now a new enemy in the USSR, Emperor Hirohito declared to the Japanese people his willingness to sign a surrender on 15 August. With half of the military government still wanting to fight on despite it all, only the emperor's intervention had allowed the possibility of peace. As Marquis Kōichi Kido, chief advisor to the emperor, later noted:

In a way it could be said that the atomic bombings and Russia's entry into the war against Japan helped to bring about the end of the war. If those things had not happened, Japan at that stage probably could not have stopped fighting.
(Holmes, 575)

Emperor Hirohito was permitted to keep his position, but after renouncing his divinity as claimed by the Shinto religion, his role was redefined as a constitutional monarch. The United States then kept occupying troops in Japan until 1952.

Dr James Bulgin, head of public history at Britain's Imperial War Museums, here summarises the ongoing discussion over the motivations and rights and wrongs of dropping the two atomic bombs:

The decision to drop the bombs on Japan at the end of the Second World War remains extraordinarily controversial right up until the present day, there were those who maintain that the war would have ended without having to use them. There are those who maintain that even if it did ultimately have a significant role in forcing Japan's hand, that simply cannot be justified by the human cost involved. But that is something which is unjustifiable under any circumstances. There are those who maintain elsewhere that it wasn't, in fact, the bombs that ended the Second World War in Asia and the Pacific, it was the Soviet Union's decision to declare war on Japan and to invade Manchuria. These are the subjects of ongoing discussion, and ultimately, the likelihood is we'll never arrive at a consensus answer on them.
(IWM)

Whatever the rights or wrongs of using a nuclear weapon, the genie was out of the bottle. The question on everyone's minds in 1945 was: what might happen next? Oppenheimer and many other prominent scientists protested nuclear fission being used for weapons and called for moves to make the control of such weapons the domain of an international body. Some scientists went further and called for the total ban of nuclear weapons. This never happened. In fact, the opposite happened. Scientists in the US continued to experiment and developed an even more powerful bomb, the hydrogen bomb, which was first tested in 1952 in the Marshall Islands. The USSR developed its own successful nuclear programme, as did several other states. Now not even aircraft were needed; atomic rockets could hit targets anywhere at the push of a button. Nuclear weapons were clearly here to stay, and the world has been holding its breath ever since in the hope that no state would use them in anger ever again.