By Jonathan Marcus
BBC Diplomatic correspondent
The nuclear deal with India enshrines a shift in US policy with far-reaching implications.
The US sees India as a powerful democratic ally in the region
It underscores the special relationship between Delhi and Washington. And it sends powerful - and in many ways contradictory - signals about the Bush administration's attitude towards the nuclear non-proliferation regime.
As far as the Bush administration is concerned, this is a win-win deal.
A friendly, democratic and powerful regional ally, India gains access to civil nuclear technology to help power its industrial growth.
At the same time India will effectively have to segregate its nuclear facilities into two programmes - one civil and the other military, with the former coming under additional international safeguards.
Not so long ago, of course, India was one of the nuclear bad-guys.
It has steadfastly refused to sign the Nuclear Non-Proliferation Treaty (NPT) and it has developed its own nuclear bomb.
The US applied sanctions against India and a battery of US legislation put India into a kind of nuclear isolation.
Hurdle one for the Bush administration is to get Congress to unpick this legislation.
Attitudes on Capitol Hill are mixed. There is a good deal of unease about the agreement on non-proliferation grounds, a fear that India is being rewarded despite its nuclear weapons programme and a belief that Washington could have struck a tougher bargain.
There are concerns, for example, that India will still be able to produce more fissile material for its bomb-making programme and thus will be able to expand its nuclear arsenal.
Nonetheless, there is also a strong tide of pro-Indian sentiment among US legislators.
Hurdle two is to persuade the 44-nation Nuclear Suppliers Group - who co-ordinate sales of nuclear technology - of the merits of the deal, where again, opinion is divided.
Here the US has already had some preliminary talks.
Some countries back the US and India. Britain, Russia and France (Paris, too, is eager to sell India nuclear technology) have all, in terms of initial positions, been positive.
Many fear that Washington is again signalling that international rules set standards that can be conveniently ignored in some cases
Ireland, Japan and the Netherlands have been far from enthusiastic.
Agreement at the group is not necessarily essential.
But its consensus view on nuclear transfers provides a useful international benchmark.
Since so much of non-proliferation policy depends upon creating as broad a diplomatic coalition as possible, it would be unwise of the US to break this tradition of consensus.
The deal could have significant regional implications, especially if India continues to develop the military side of its nuclear programme.
China's only comment so far has been a rather restrained call for any US-India co-operation to be in line with the rules of the global non-proliferation regime.
Though there are some strategic experts in Beijing who are carefully watching the burgeoning US-India relationship for any sign that India is being turned into a regional bulwark against China.
It is in the arms control community - especially among US experts - that the deal has caused most concern.
Many fear that Washington is again signalling that international rules - like the NPT - set standards that can be conveniently ignored in some cases, while the Bush administration vigorously tries to apply them in others.