US President Richard Nixon and the Soviet Union's General Secretary Leonid Brezhnev reached the first arms pact between the two nuclear superpowers in 1972 when they simply agreed not to increase their stocks of weapons further.

The first treaty to envisage a reduction in arsenals followed in 1979 but the number of weapons did not begin to come down until the 1980s.

Each successive treaty took years to implement, with the targets of 1991's Start I agreement only being reached in December 2001.

Start II was due to be enforced by 2007 though it has now been superseded by the Moscow Treaty and the Start III plans of Bill Clinton and Boris Yeltsin will be forgotten.

The 1991 break-up of the Soviet Union left nuclear weapons in the hands of four new republics, all of which later said they would be bound by the earlier treaties.

Belarus and Kazakhstan transferred their nuclear warheads to Russia and destroyed the accompanying delivery vehicles while Ukraine has also moved its warheads to Russia and is continuing to eliminate the associated missile silos and heavy bombers.

While the Start treaties included measures to destroy the methods of delivering nuclear weapons - such as bombers, submarines and launchers - the US did not want these addressed in the Moscow Treaty.

So, for instance, the US could reduce its strategic arsenal to one launcher with one warhead while putting nine other warheads into storage.

That would then count as one weapon, though in theory the other nine warheads could be brought out of reserve and added to the launcher which has been kept in functioning order.