At 8.15am on August 6, 1945 a US B29 bomber flew over the Japanese city of Hiroshima at 27,800 feet, released its load, turned sharply to the right and roared off. The atomic bomb, 1,000 times more powerful than other explosives, fell to 1,900 feet and exploded.

A huge fireball sent a pillar of smoke thousands of feet upwards then outwards, in the shape of a giant mushroom. The blast flattened almost five square miles - 60 per cent of the city. More than 78,000 of its 348,000 population were killed, and 51,000 were injured or missing.

It was dubbed the absolute weapon and the president of the US, Harry S Truman, returning from the Potsdam conference, declared it “the greatest thing in history”.

Three days later a second, bigger bomb was dropped on another Japanese city, Nagasaki, killing or wounding about 75,000 people. Days later Japan surrendered.

The two atom bombs ended the Second World War. But they ignited another potentially more deadly battle - the nuclear arms race. While whole swathes of central and eastern Europe fell under communist control, the Americans drew up a plan to use their atomic advantage to win a third world war, by wiping out 20 Russian cities.

But within three years the Soviet Union had tested its own atom bomb and the race was on. A-bombs gave way to hydrogen bombs, 1,000 times more powerful, and by 1972 a single US bomber carried more destructive power than all the weapons used in all the wars in human history. But it was the advent of missiles, following the Soviet Union’s1957 launch of Sputnik, the first satellite, that took the world a hair-trigger away from obliteration, because intercontinental missiles, like these Titans in the US, dramatically cut the time between the launch of an attack and impact.

Launched into space, they could hit an enemy city within half an hour. Submarine launches close to the Soviet or US coast could cut that time to six minutes.

It was the madness of mutually assured destruction that deterred each superpower from launching a first strike. But this balance of terror was upset in the 1980s by Ronald Reagan, who tried to revive the idea of an anti-missile shield - banned in 1972 - to zap incoming missiles and ratcheted up the arms race. Although nominally defensive, the shield’s unreliability made it more suited to aiding an attack, by mopping up the few Soviet missiles that would survive an American first strike.

Reagan later admitted the “Star Wars” plan was a ploy to coax the Soviet Union into spending money it could not afford on missile technology. The gamble paid off. The Soviet economy, with 37 per cent of government spending going on defence, went belly-up and communism collapsed. But although many agreements on disarmament have been signed since the fall of the Berlin Wall, the development of nuclear arsenals by so-called “rogue states” and George Bush’s plans for a new Star Wars shield suggest the nuclear arms race will run and run.

Brendan O’Malley Weblinks Account of the Hiroshima blast: http:mothra.rerf.or.jpENG HiroshimaContents.html Nagasaki bomb: www.thehistorynet.com AviationHistoryarticles1997 01972_text.htm FAQs on nuclear weapons: www.fas.orgnukehewNwfaq Nfaq7.html Disarmament briefings: www.cnduk.orgbriefingindex.htm