By Jonathan Marcus
BBC diplomatic correspondent
Both the US and Russia were keen to reduce their arsenals
This agreement marks a significant foreign policy breakthrough for President Barack Obama.
It is the first comprehensive deal on nuclear arms since the Strategic Arms Reduction Treaty - known as Start - signed back in July 1991.
That treaty expired at the end of last year and US and Russian negotiators have been working flat out to try to get the new successor agreement in place.
It has not always been an easy set of negotiations.
The Russians, with an ageing nuclear arsenal, badly wanted a deal to peg their respective arsenals at lower levels
There have been complex discussions about technical matters - sharing telemetry from missile flights, for example.
At times it looked as though Moscow's continuing unease about US missile defence plans might derail the talks altogether.
However, both sides want the predictability that this new treaty will bring.
The Russians, with an ageing nuclear arsenal, badly wanted a deal to peg their respective arsenals at lower levels.
In diplomatic terms Russia also rather likes strategic arms talks with the Americans. This is one of the few areas, after all, where Washington and Moscow sit at the same table as near-equals.
The reductions in this new treaty in terms of strategic warheads sound significant - but they leave both sides still with many more weapons than they need to deter the other
The new agreement is a real arms control treaty in the sense that it includes a panoply of verification and monitoring measures that give such texts their real force.
An interim arms deal, signed by Presidents George W Bush and Vladimir Putin some eight years ago, contained none of these provisions - elements that arms control experts see as essential in providing the transparency and confidence needed between the two nuclear-armed superpowers.
The reductions in this new treaty in terms of strategic warheads sound significant - but they leave both sides still with many more weapons than they need to deter the other.
Ambition v pragmatism
Nonetheless, the new treaty, to be signed in Prague in early April, marks the first step in the US president's ambitious arms control agenda.
In a keynote speech in the Czech capital almost a year ago, Mr Obama set out his vision of moving towards a world without nuclear weapons.
This was ambition tinged with pragmatism. He realised that such a world would probably not be achieved in his lifetime. But this arms control treaty is an important start.
The new treaty also sends an important signal ahead of a crucial meeting in May to review the nuclear non-proliferation treaty - the cornerstone of efforts to prevent the spread of nuclear weapons.
Both Washington and Moscow want to be seen to be reducing their nuclear arsenals - something non-nuclear nations demand if the whole non-proliferation regime is to work effectively.
The agreement also marks an improvement in ties between the US and Russia.
Mr Obama is determined to, as the Americans put it, "reset" this relationship.
Too often, the mistrust of the Cold War years seems to have crept back in and Russia, for all the US concerns about some of its policies, is seen as a crucial partner in dealing with the pressing problems of a globalised world.
This new arms reduction treaty is not an end in itself.
The treaty is seen in Washington as part of an ambitious programme
The Obama administration certainly sees this as the start of an ambitious new process, involving agreements on safeguarding nuclear materials, bolstering non-proliferation efforts and securing further and deeper weapons cuts in the future.
It is how these pieces of the atomic jigsaw are put together that constitutes the real test of Mr Obama's nuclear diplomacy.
It is made ever more important by the failure, so far, to contain Iran and North Korea's nuclear ambitions and by the renewed popularity of civil nuclear power, which is spreading into unstable parts of the world, raising the spectre of more countries flirting with nuclear weapons ambitions.