The First Atomic Bomb in Combat Was Dropped on Hiroshima at 8:15 a.m. on 6 August 1945 and Killed Approximately 140,000 People by the End of the Year

The American Manhattan Project had begun in 1942 under the military scientific direction of Major General Leslie Groves and the civilian scientific direction of J. Robert Oppenheimer. The project had culminated in the first nuclear-fission detonation at the Trinity test in southern New Mexico on 16 July 1945 — three weeks before Hiroshima.

The Japanese Empire by July 1945 was militarily defeated but had not formally surrendered. The American strategic alternatives presented to President Harry Truman were continued conventional bombing and naval blockade (with estimated additional American casualties of approximately 250,000 across 18 months), invasion of the Japanese home islands (with estimated additional American casualties of up to 500,000), and use of the new atomic weapon.

Truman authorised the atomic bombing on 25 July 1945. The first target was the city of Hiroshima — selected because it had not been previously bombed (and therefore provided clean damage assessment data) and because it contained the Second General Army headquarters of the Imperial Japanese Army.

6 August 1945

The B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets of the 509th Composite Group, took off from the American airbase at Tinian Island in the Mariana Islands at 2:45 a.m. local time on 6 August 1945. The bomb in the bomb bay — code-named Little Boy — was a 4-ton uranium-235 gun-type fission weapon containing approximately 64 kg of enriched uranium. The design had not been tested at full yield (only the plutonium implosion design had been tested at Trinity); the Hiroshima drop was the first uranium-bomb detonation.

The Enola Gay arrived over Hiroshima at approximately 31,000 feet altitude at 8:14 a.m. local time. The bomb was released. Forty-three seconds later — at 8:15 a.m. and 17 seconds local time — Little Boy detonated at approximately 580 metres above the Shima Surgical Clinic in central Hiroshima.

What happened on the ground

The detonation produced approximately 15 kilotons of TNT equivalent yield. The peak temperature at the fireball was approximately 7,000°C. The blast wave radiated outward at approximately 440 metres per second.

Within the first second after the detonation:

— Substantial everyone within approximately 500 metres of the hypocentre died instantly from thermal radiation. Their bodies vaporised; some left silhouettes scorched into stone walls. About 70,000 people were within this inner zone. — Substantial everyone within approximately 1,500 metres was severely burned by the thermal radiation. — Substantial reinforced concrete buildings within approximately 2 km were structurally damaged but survived. — Substantial wooden buildings within approximately 3 km were crushed by the blast wave.

The Hiroshima fire that followed the blast burned for approximately 18 hours. Approximately 70 percent of the city’s buildings were destroyed by the combined blast and fire.

The death toll

The initial estimates ran:

— Killed by 6 August 1945: approximately 70,000-80,000 — Killed by end of 1945: approximately 140,000 (including acute radiation sickness deaths) — Killed by 1950: approximately 200,000 (including late radiation effects)

The population of Hiroshima had been approximately 350,000 in August 1945. Approximately 40 percent of the city’s population died within five months.

Nagasaki and surrender

The second atomic bomb — Fat Man, a plutonium implosion weapon — was dropped on Nagasaki on 9 August 1945 at 11:02 a.m. The Nagasaki yield was approximately 21 kilotons. The mountainous local topography limited the damage radius. Approximately 40,000 died within the first day; approximately 80,000 by end of 1945.

The Japanese Emperor Hirohito recorded a radio surrender announcement on the morning of 15 August 1945. The broadcast — the first time any Japanese emperor had addressed the public directly — used the famous euphemism that “the war situation has developed not necessarily to Japan’s advantage.” The formal surrender was signed aboard the USS Missouri in Tokyo Bay on 2 September 1945.

What followed

The post-war American occupation of Japan demilitarised the country and imposed the 1947 Constitution that renounced war as a sovereign right. The post-1945 Japan became a pacifist constitutional democracy under American security protection.

The Hiroshima Peace Memorial Park preserves the ruins of the Hiroshima Prefectural Industrial Promotion Hall — the sole structural survivor of the hypocentre area, known as the A-Bomb Dome. The dome remains in its 6 August 1945 damaged condition as a UNESCO World Heritage Site.

The nuclear weapons that the 1945 detonations introduced still exist. As of 2026 approximately 12,500 nuclear warheads are held by nine states. The Hiroshima and Nagasaki bombs remain the only two nuclear weapons used in combat.