The Russian and American presidents are due to sign a long-awaited nuclear weapons pact in the Czech capital that will replace the 1991 Strategic Arms Reduction Treaty (Start).
The new treaty marks a milestone in the arms control process, as the BBC’s Diplomatic Correspondent Jonathan Marcus reports from Prague.
This is really the first proper nuclear arms reduction treaty of the post-Cold War era.
It scales back the US and Russian nuclear arsenals – though by how much is an interesting point which I will come to in a moment.
It contains a full battery of verification measures to ensure compliance and it is equally important for the diplomatic signals that it sends in terms of relations between Moscow and Washington and for President Barack Obama’s wider nuclear disarmament goals.
Managing the nuclear rivalry between Russia and the United States was a central concern of the Cold War years.
Tensions may have eased but reducing the size of each country’s nuclear arsenals and ensuring that the new limits can be properly verified introduces a useful predictability in relations between Washington and Moscow.
The new Start follow-on agreement, as it is clumsily being referred to – perhaps the Prague Treaty might be a better name – reduces each country’s arsenals to some 1,550 deployed nuclear warheads.
At present the US is thought to deploy some 2,200 warheads and the Russians somewhere between 2,600 and 2,700.
Under the new treaty each side will be allowed no more than 700 deployed ballistic missile launchers or heavy bombers equipped to deliver nuclear bombs.
How significant is this reduction?
US spokesmen claim that the new limit of some 1,550 warheads is about 30% lower than the target of 2,200 set by the 2002 Bush-era Strategic Offensive Reductions Treaty.