In 2005, the sixtieth anniversary of the bombing of Hiroshima and Nagasaki, I nominate the principles of fission that underlay the atomic bomb, and the principles of fusion that informed the subsequent development of nuclear weapons.

On 6 August 1945, above the undefended port of Hiroshima, US president Harry Truman's Democratic administration - actively supported by UK prime minister Clement Attlee's Labour government - used the convertibility of mass to energy, advanced by Albert Einstein in his famous equation E = mc², to catastrophic effect. Einstein had not foreseen the possibility of a fission chain reaction, leading to an unprecedented release of energy - 'that never occurred to me', he is reported to have said. But at the behest of the Hungarian scientist Leo Szilard, Einstein wrote to the US president Franklin Delano Roosevelt on 2 August 1939 that nuclear chain reactions in a large mass of uranium meant 'a single bomb...exploded in a port, might very well destroy the whole port together'.

Tokyo had been targeted for the Bomb, at one time. In a striking turn of events, Kyoto was eventually allowed off, because of its stature as a 'cultural' city. In the USA, the $2billion Manhattan Project went hand in hand with the putting out of diplomatic peace feelers to Japan's emperor, the drawing up of plans for invasion, and the continued non-nuclear bombing of Japan. Indeed, by the summer of 1945, the USA had few Japanese targets left to attack.

Nevertheless, more than 200,000 were made to die in Hiroshima and Nagasaki. Earlier, the physicist Robert Oppenheimer, director of the Manhattan Project, had enthused about the 'profound psychological impression' that the 'bright luminescence' of an atomic explosion would make upon Japanese civilians. Delivered covertly by a single aircraft, a single bomb could lay waste to an entire city. Not just for the Japanese, but for the whole world, fission bombs turned out to be the earliest and most terrible examples of what we would today recognise as the tactic of shock and awe.

The fat, rather unstable nuclei of the isotope ²³⁵U, which is found in 0.6 per cent of the naturally occurring heavy metal uranium, are relatively easy to split with a neutron, compared with the nuclei of most other elements. Moreover, apart from the release of lethal gamma radiation, the broader release of energy that attends such a split is powerful enough to fuse the nuclei of various hydrogen isotopes, form helium nuclei and release even larger amounts of energy. In this manner, atomic fission triggers nuclear fusion, in what used to be known as the hydrogen bomb.

In April 1979, the Democratic administration of US president Jimmy Carter cut off assistance to Pakistan, because of Pakistan's efforts to enrich the proportion of naturally occurring uranium that contains ²³⁵U. Since then, the USA, the UK and the United Nations have used all aspects of uranium extraction and refinement - along with the production of fissile plutonium, in nuclear reactors - as a means of challenging the sovereignty of Pakistan, India, North Korea and of course, Iraq. More recently, the UK, France and Germany have tried to police Iranian fission, while the United Nations has upbraided Brazil and South Korea for their experiments. A discreet veil has been passed over Israel's 200 nuclear warheads, although even here, there are fears of proliferation.

Fission and fusion lie behind the conclusion of the Second World War, behind the balance of terror in the Cold War, and behind the West's interventions in the Third World in general and in Iraq in particular. In fact, as we now know of Iraq, it takes a substantial industrial capacity to make and weaponise nuclear missiles. Meanwhile, efforts to build the $10billion International Thermonuclear Experimental Reactor - the world's first demonstration fusion plant, and one that is due to generate electricity for about 20 years from 2014 - are foundering in a dispute between Japan and the European Union, over where the device should be located.

From 1945 to 2005, fission and fusion - together with fear of and ignorance about them - have had a lot to do with the changing course of international relations. If I could teach everyone in the world just one thing about science, then it would be the possibilities and limitations that surround these two phenomena.

James Woudhuysen is coauthor of Why Is Construction So Backward? (buy this book from Amazon (UK) or Amazon (USA)), and coeditor of Einstein: The First 100 Years (buy this book from Amazon (UK) or Amazon (USA)). See his website.