Japan marks the anniversary of the US atomic bombing of Hiroshima on Thursday, 81 years after the attack that ushered in the nuclear age and changed the course of modern warfare.
Three days after Aug. 6, 1945, the US dropped a second atomic bomb on Nagasaki, setting off a series of events culminating in Japan announcing its surrender on Aug. 15 and bringing World War II to an end in Asia.
The bombings remain the only use of nuclear weapons in war. More than eight decades later, they continue to frame global arguments over nuclear deterrence, arms control, disarmament and the humanitarian consequences of weapons capable of destroying entire cities.
At 8.15 a.m. (2315GMT) on Aug. 6, 1945, a US B-29 bomber named Enola Gay dropped an atomic bomb, codenamed "Little Boy," over Hiroshima, a city populated by about 350,000 people at the time.
The bomb exploded above the city, producing an massive shockwave, extreme heat and radiation. Much of central Hiroshima was flattened within seconds.
By the end of 1945, around 140,000 people are estimated to have perished in the initial blast in Hiroshima, a military and industrial city, as well the burns, trauma and radiation-related illness that claimed the lives of many survivors.
On Aug. 9, 1945, the US dropped a second atomic bomb, codenamed "Fat Man," on Nagasaki.
Nagasaki was an important port and industrial city, with shipbuilding and arms-related facilities. The city's mountainous terrain limited the spread of destruction compared with Hiroshima, but the impact was nonetheless catastrophic.
The bomb killed tens of thousands instantly or in the following weeks and months. By the end of 1945, around 74,000 people are estimated to have died in Nagasaki.
The US said the bombings were intended to force Japan's surrender and avoid a costly invasion of the Japanese mainland.
By the summer of 1945, Germany had already surrendered, but the war in the Pacific continued. US forces had captured islands closer to Japan after some of the war's bloodiest battles, including Iwo Jima and Okinawa.
Supporters of the decision argue that the bombs brought the war to a faster end and prevented even greater casualties among US troops and Japanese civilians.
Critics argue that Japan was already weakened, that alternatives could have been pursued, and that the bombings caused unacceptable mass civilian suffering.
The debate remains one of the most contested moral and historical questions of World War II.
The atomic bombs released enormous energy through nuclear fission, the splitting atoms to create a blast far more powerful than conventional explosives.
The destruction came in several forms: an intense shockwave, extreme heat, fires, and radiation.
People near the hypocenter were killed instantly. Others died later from burns, injuries, radiation sickness and longer-term illnesses. Survivors also faced trauma, displacement, stigma and health problems that lasted for decades.
The attacks showed for the first time that one weapon could devastate a city in seconds.
Hibakusha is the Japanese term for survivors of the atomic bombings.
Many hibakusha spent their lives testifying about what they saw and experienced, warning against the use of nuclear weapons and calling for disarmament.
Their accounts have become central to global anti-nuclear movements, especially as the survivor population ages. For many campaigners, the anniversaries of Hiroshima and Nagasaki are not only memorial events, but reminders of the human cost of nuclear war.
The bombings marked the beginning of the nuclear age.
After World War II, the US was briefly the only nuclear-armed state, but the Soviet Union tested its first atomic bomb in 1949. Britain, France, China and other states later developed nuclear weapons, creating a global arms race.
The Cold War was shaped by the threat of nuclear conflict, particularly between the US and the Soviet Union. The doctrine of nuclear deterrence rested on the idea that nuclear-armed states would avoid direct war because the consequences would be catastrophic for all sides.
At the same time, the bombings of Hiroshima and Nagasaki helped fuel global disarmament campaigns and later treaties aimed at limiting nuclear weapons and preventing their spread.
The anniversary comes at a time when nuclear risks remain a major concern.
Several countries possess nuclear weapons, arms control agreements have weakened in recent years, and conflicts involving nuclear-armed states or their allies have revived fears of escalation.
For nuclear-armed governments, such weapons are often defended as tools of deterrence. For disarmament advocates, Hiroshima and Nagasaki show that any use of nuclear weapons would have unacceptable humanitarian consequences.
The divide between those views remains at the center of global nuclear politics.
Hiroshima and Nagasaki remain symbols of both wartime devastation and the struggle to prevent nuclear war. The anniversaries are observed each year with memorial ceremonies, moments of silence and renewed calls for peace.
