The atomic bombings of Hiroshima and Nagasaki were two nuclear attacks carried out by the United States against Japanese cities during the final stage of the Second World War. The first bomb, known as Little Boy, was dropped on Hiroshima on 6 August 1945 from the B 29 bomber Enola Gay. The second, known as Fat Man, was dropped on Nagasaki on 9 August from the B 29 Bockscar.
Hiroshima was struck with a uranium based gun type weapon, while Nagasaki was struck with a plutonium based implosion weapon. The explosions caused enormous immediate destruction, widespread fires, severe burns, blast injuries and radiation exposure. Many people died immediately, while others died during the following days, weeks, months and years from injuries and radiation related illness. The bombings occurred alongside other decisive developments, including the Soviet declaration of war against Japan on 8 August and the Soviet invasion of Japanese held Manchuria on 9 August. The combination of these events contributed to Japan's decision to surrender.
Hiroshima was a major Japanese military and industrial city with an important army headquarters and transportation infrastructure. The atomic bomb detonated above the city, producing an enormous blast and thermal pulse. Buildings over a wide area were destroyed or severely damaged, fires spread rapidly and tens of thousands of people died in the initial period.
The exact death toll cannot be established with complete precision because records were destroyed, many victims were civilians, and later deaths from injuries and radiation continued for years. Modern estimates commonly place the number of deaths in Hiroshima by the end of 1945 at roughly 140,000, although estimates vary according to methodology and the definition of radiation related mortality. The physical effects included blast pressure, extreme heat, firestorms, structural collapse and radiation. Survivors experienced severe burns, traumatic injuries and later symptoms associated with radiation exposure.
Nagasaki was attacked three days later. The original target for the second mission was Kokura, but cloud and visibility conditions prevented the crew from obtaining the required visual identification. The aircraft therefore proceeded to Nagasaki, where the bomb was dropped. Nagasaki was an important industrial centre containing shipyards, factories and military facilities. The geography of hills and valleys affected the distribution of blast and heat, so the destruction pattern differed from Hiroshima.
The bomb nonetheless caused catastrophic damage and tens of thousands of deaths. As with Hiroshima, the exact number of victims is uncertain. Estimates commonly place deaths by the end of 1945 at around 70,000, although figures vary. Survivors experienced long term health effects, including increased risks of certain cancers and other radiation associated conditions.
The physical mechanism of the weapons was fundamentally different from conventional explosives. Nuclear weapons release energy through nuclear fission, in which atomic nuclei split and release additional neutrons that can cause further fission in a rapidly expanding chain reaction. The resulting energy appears as blast, thermal radiation and ionising radiation.
Little Boy used highly enriched uranium in a relatively simple gun type design, while Fat Man used plutonium and an implosion mechanism requiring carefully shaped conventional explosives to compress the plutonium core. The scientific achievement depended on the Manhattan Project, an enormous American led programme involving scientists, engineers, industrial workers and military organisations. Research took place at multiple locations, including Los Alamos, Oak Ridge and Hanford. The successful Trinity test on 16 July 1945 demonstrated that the new technology worked before it was used against Hiroshima.
The human consequences extended far beyond the immediate explosions. Survivors, later known as hibakusha, experienced burns, injuries, psychological trauma and radiation related illnesses. Radiation exposure could damage bone marrow and other tissues, while longer term studies identified increased risks of leukaemia and several solid cancers. Pregnant women and children were particularly vulnerable to some effects of radiation. The social consequences included bereavement, homelessness, destruction of medical facilities and disruption of food and water supplies.
Japanese survivors sometimes faced discrimination because of misconceptions about radiation and fears concerning illness or heredity. The bombings therefore cannot be understood solely as military events. They became major events in the history of medicine, nuclear science, international relations, ethics and the study of the long term effects of radiation on human populations.
The bombings took place within a broader American campaign of conventional bombing against Japanese cities. Earlier incendiary raids had already destroyed large urban areas and killed enormous numbers of civilians. The Tokyo firebombing of March 1945 was particularly destructive. The atomic bombs differed because a single aircraft could cause destruction on a scale previously requiring very large formations of conventional bombers, while also producing ionising radiation.
The bombings therefore represented a new category of warfare even though they occurred within an existing campaign of strategic bombing. The decision to use nuclear weapons must consequently be understood within the military context of 1945, when Japanese cities were already being attacked and Allied leaders were considering how to end a war in which Japan had not accepted unconditional surrender.
Japan's political leadership did not immediately surrender after Hiroshima. The Soviet Union entered the war against Japan on 8 August, and Soviet forces launched a large offensive against Japanese forces in Manchuria on 9 August. Nagasaki was bombed on the same day. These developments created a rapidly changing strategic situation. Within Japan, the Supreme War Council remained divided over whether to accept surrender, and Emperor Hirohito played a decisive role in breaking the deadlock.
Japan announced its acceptance of the Allied surrender terms on 15 August 1945. The formal surrender occurred on 2 September aboard USS Missouri. The exact relative importance of the atomic bombings and Soviet entry remains a major subject of historical debate, and historians differ over how each factor affected Japanese decision making.
The bombings also transformed world history because they demonstrated that nuclear weapons could be used in warfare and initiated the nuclear age. The United States retained a nuclear monopoly only briefly, as the Soviet Union developed and tested its own atomic bomb in 1949. Nuclear weapons subsequently became central to the Cold War strategic relationship between the United States and Soviet Union. Hiroshima and Nagasaki also became sites of international remembrance and anti nuclear activism.
Historians continue to study the bombings through military, diplomatic, ethical, scientific and humanitarian perspectives. Any comprehensive understanding must distinguish the documented physical consequences from later arguments about whether their use was necessary. The events remain among the most consequential and contested episodes of twentieth century history because they simultaneously ended a catastrophic war, caused immense civilian suffering and introduced a weapon whose existence transformed international security.
