1. The road to August 1945
By the summer of 1945 Germany had surrendered and Japan was fighting alone. American bombers had already burned most large Japanese cities with incendiary raids, the navy had cut off imports of food and fuel, and an invasion of the home islands was planned for November. Japan's leaders were divided between those seeking a negotiated end and those determined to fight on.
In the United States, the Manhattan Project had been building an atomic bomb since 1942, first out of fear that Nazi Germany would get one. On 16 July 1945 the first nuclear device in history, a plutonium implosion bomb nicknamed "the Gadget", was tested at the Trinity site in the New Mexico desert. It released about 21 kilotons of energy. Ten days later the Allies issued the Potsdam Declaration demanding Japan's unconditional surrender. Japan did not accept it.
Two bombs were shipped to Tinian, a small island in the Northern Marianas about 2,500 km south of Japan, where the 509th Composite Group and its specially modified B-29 bombers were waiting.
2. The two bombs
Little Boy
A gun-type bomb: a small piece of uranium-235 was fired down a barrel into a larger piece, forming a supercritical mass. It was simple enough that it was never tested before use. It weighed about 4,400 kg, was 3 m long, and exploded with a force of roughly 15 kilotons, the equivalent of 15,000 tonnes of TNT. Only about 1.4 percent of its 64 kg of uranium actually fissioned.
Fat Man
An implosion bomb, the same design as the Trinity device: a sphere of plutonium-239 the size of a grapefruit was crushed by a shell of precisely shaped conventional explosives. It was heavier and rounder, about 4,700 kg and 1.5 m across, and more powerful, at roughly 21 kilotons. Plutonium was easier to produce than enriched uranium, so this design became the model for the bombs that followed.
3. Hiroshima, Monday 6 August 1945
Hiroshima was a city of about 350,000 people on a flat river delta, an army headquarters and a port that had so far been spared from conventional bombing. That morning the sky was clear.
- Take-off from Tinian. The B-29 Enola Gay, piloted by Colonel Paul Tibbets and named after his mother, lifts off with Little Boy on board, escorted by two other B-29s carrying instruments and cameras. The bomb is armed in flight.
- Air-raid alert, then all clear. A weather plane passes over the city. The alert is lifted, and people go back to their morning. Many are outdoors, walking to work or school.
- Detonation. The bomb falls for 43 seconds and explodes about 600 m above Shima Hospital, near the T-shaped Aioi Bridge that was the aiming point. The flash is seen 100 km away.
- Heat. The fireball reaches several thousand degrees at the surface. People within about 1 km are killed instantly; skin is burned out to 3.5 km. Shadows of people and objects are left etched on stone and walls.
- Blast. The shock wave flattens almost everything within 1.6 km and damages buildings out to 3 km and more. About 70 percent of the city's buildings are destroyed. Around 13 km² of the city is wiped out.
- Firestorm. Thousands of small fires merge into one that burns for hours. Black rain, radioactive soot mixed with water, falls on the north-west of the city.
- The aftermath. Of 45 hospitals, 3 still function. More than 90 percent of the city's doctors and nurses are dead or injured. Survivors walk out of the ruins with their skin hanging from their arms.
Between 70,000 and 80,000 people died on the day. By the end of 1945 the toll had risen to somewhere between 90,000 and 146,000 as burns, injuries and radiation sickness took their course. The uncertainty is real: the city's records burned with it.
Sixteen hours later, President Truman announced to the world what the weapon was.
4. Nagasaki, Thursday 9 August 1945
The second mission almost did not happen as planned. The B-29 Bockscar, flown by Major Charles Sweeney, took off from Tinian with Fat Man before dawn. Its primary target was the city of Kokura, but Kokura was hidden by cloud and by smoke from a conventional raid on a neighbouring city. After three passes and with fuel running low, the crew turned to the secondary target: Nagasaki.
Nagasaki was also cloudy. Through a brief gap the bombardier released the bomb at 11:02. It exploded about 500 m above the Urakami valley, roughly 3 km north-west of the intended aiming point in the city centre. That miss, and the hills that enclose the valley, kept the destruction from spreading across the whole city. Even so, the Urakami district, its cathedral, the Mitsubishi steel and arms works, and the medical college were destroyed.
About 40,000 people died on the day, and between 60,000 and 80,000 by the end of the year, out of a population of around 260,000. Fat Man was the more powerful bomb, but the terrain and the missed aim point meant that Nagasaki suffered fewer deaths than Hiroshima.
On the same day the Soviet Union declared war on Japan and invaded Manchuria. Historians still debate which of the two shocks weighed more on the Japanese government.
5. Radiation and the survivors
Nuclear weapons kill in three ways: heat, blast and radiation. The first two did most of the killing in August 1945. The third kept killing for decades.
People who received large doses of gamma and neutron radiation in the first minute developed acute radiation sickness within days: nausea, hair loss, bleeding gums, then a collapse of the bone marrow. Many who seemed to have escaped with light injuries died in the second and third weeks. From about 1947 leukaemia rates among survivors rose sharply, peaking in the early 1950s; solid cancers followed and are still slightly elevated in the surviving population today.
The survivors are known in Japan as hibakusha, "bomb-affected people". More than 100,000 were still alive in 2024. Many faced discrimination for years, feared as contagious or as unfit to marry. Their testimony, and that of children like Sadako Sasaki, who folded paper cranes as she died of leukaemia at twelve, has shaped how the world remembers the bombings.
6. Surrender and what came after
- Japan surrenders. Emperor Hirohito addresses the nation by radio for the first time, citing "a new and most cruel bomb". The formal surrender is signed on 2 September aboard the USS Missouri in Tokyo Bay.
- The Soviet bomb. The Soviet Union tests its first nuclear weapon. The arms race begins.
- The hydrogen bomb. Thermonuclear weapons make Hiroshima look small: the first US test is 700 times Little Boy, and the Soviet Tsar Bomba of 1961 is more than 3,000 times.
- Remembrance. The Hiroshima Peace Memorial Park opens around the skeletal Atomic Bomb Dome, one of the few structures left standing near ground zero. The Dome becomes a UNESCO World Heritage site in 1996.
- Nine countries hold about 12,000 nuclear warheads. Most are many times more powerful than the two used on Japan. Nobody has used one in war since 9 August 1945.
7. How 1945 compares with today's weapons
The bombs dropped on Japan were small by modern standards. The table uses the same scaling model as the simulator: destruction grows with the cube root of the yield, so a weapon 1,000 times more powerful has a blast radius only about 10 times larger.
| Weapon | Yield | × Hiroshima | Buildings collapse | 3rd-degree burns |
|---|---|---|---|---|
| Little Boy, Hiroshima 1945 | 15 kt | 1× | 1.2 km | 1.9 km |
| Fat Man, Nagasaki 1945 | 21 kt | 1.4× | 1.3 km | 2.2 km |
| W87, US Minuteman III today | 300 kt | 20× | 3.3 km | 6.5 km |
| Topol-M, Russian ICBM today | 800 kt | 53× | 4.5 km | 9.7 km |
| Tsar Bomba, 1961 test | 50 Mt | 3,333× | 18 km | 53 km |
Sources and further reading
- Hiroshima Peace Memorial Museum and Nagasaki Atomic Bomb Museum, official histories and casualty estimates.
- Radiation Effects Research Foundation (RERF), the joint Japan–US study of survivors' health since 1947.
- John Hersey, Hiroshima (1946), the account of six survivors first published in The New Yorker.
- Richard Rhodes, The Making of the Atomic Bomb (1986).
- Samuel Glasstone and Philip Dolan, The Effects of Nuclear Weapons (1977), the source of the scaling laws used on this site.
Casualty figures for both cities are ranges because records were destroyed, many victims were never identified, and deaths from radiation continued for years. Different institutions count differently; the ranges above cover the most cited estimates.