

On 6 August 1945, the US attacked the Japanese city of Hiroshima with an atomic bomb in a bid to end the second world war. Seventy years after the devastating power of nuclear weapons was first demonstrated, nine states retain them in their arsenals

The Manhattan Project

US atomic weapons research began after nuclear fission was discovered by German scientists in 1938, prompting fears of a Nazi bomb. After Japan attacked Pearl Harbor in 1941, work centred on the Manhattan Project, led by Robert Oppenheimer at Los Alamos in New Mexico. Germany had already surrendered when the first nuclear weapon test took place on 16 July 1945, but war in the Pacific continued.

The attacks

After the successful test US president Harry Truman authorised the use of two weapons against Japan, arguing it would be a quicker and less bloody way to secure surrender than an invasion. There was no capitulation after the first bomb, codenamed Little Boy, destroyed more than 10 sq km of Hiroshima on 6 August 1945. Three days later the more powerful Fat Man device hit Nagasaki.

The casualties

Estimates of people killed in the immediate aftermath of the two bombings and the months that followed range as high as 246,000. Many of the survivors suffered horrific burns and the enduring effects of radiation illnesses. With more attacks planned by the US, Japan surrendered on 15 August.

Cold war follows world war

The USSR, which had spies in the Manhattan project, tested its first nuclear bomb in 1949. Increasingly powerful thermonuclear devices were tested in remote parts of the world, culminating with the Soviet Tsar Bomba which produced an explosion visible 1,000 kilometres away and a mushroom cloud taller than Everest.

Mutually assured destruction

Throughout the 1960s, the superpowers developed huge arsenals. According to the doctrine of mutually assured destruction (MAD), this made nuclear war unlikely as neither side could ever completely destroy the other’s ability to retaliate. The 1970 non-proliferation treaty was designed to restrict the capability to the existing nuclear powers, but other states were already pursuing their own programs.

Disarmament and proliferation

The collapse of the Soviet Union in 1991 removed the cold war’s uneasy certainty. There are now nine known nuclear powers, chief among them the US and Russia, which retain formidable stockpiles even after substantial disarmament. Only South Africa has ever independently developed and then relinquished nuclear weapons. Ukraine surrendered its Soviet weapons in 1994 in exchange for a guarantee of its territorial integrity.

The doomsday clock

Since 1947, the Bulletin of the Atomic Scientists has been publishing a yearly assessment of the risk of global catastrophe in the form of a clock counting down to midnight. In 2015, with climate change now included as a risk, it stands at three minutes to midnight, as existing powers upgrade their arsenals. Ongoing risks to humanity include further proliferation, a nuclear terrorist attack and the problem of radioactive waste.