Start background

The original Strategic Arms Reduction Treaty was signed in 1991 in Moscow by US President George Bush and Soviet leader Mikhail Gorbachev.

It came into force three years later, delayed by the collapse of the USSR, and led ultimately to the removal of about 80 percent of all strategic nuclear weapons.

Since the original START treaty each country has reduced their nuclear capacity to a combined total of 1,600 ballistic missiles and 6,000 nuclear warheads.

The new agreement now seeks to go further – cutting missile numbers by half and warheads by two-thirds over the next seven years.

Russia and the US can also now visually inspect each others nuclear arsenal.

The warhead figures are lower than the number agreed when the treaty was revised in 2002.

A new deal was meant to signed before the original START treaty expired in 2009 but negotiations only concluded in Prague in April this year.

President Obama has been under pressure to ratify the treaty by the end of this year – despite opposition by Republicans. Moscow has said that any attempts to change the treaty now would effectively kill it.

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