By Nick Childs BBC Pentagon correspondent

Much of the heat that surrounded the debate over the ABM treaty last year has dissipated after it became increasingly clear that the Bush administration was going to abandon it.

The opposition was muted by the fallout from the 11 September attacks, and there has been a grudging acceptance by Russia that President George W Bush was determined to press ahead with his plans anyway.

That is not to say that the Russians, or even some of Washington's Nato allies, are happy.

And there remains a climate of uncertainty about what the abandonment of the ABM treaty, and the US pursuit of missile defence, will mean for strategic stability and weapons proliferation.

The debate

Some critics argue that the US withdrawal is more political than practical, and that it could have carried on missile defence testing for some years without breaching treaty restrictions.

Doubts still linger about the effectiveness of the system

They also argue that the actual threat posed by "rogue" states acquiring missiles that could reach the United States is still years away, and that diplomacy rather than defences is the better way to deal with the issue of missile proliferation.

Work is about to start on a missile interceptor test bed at Fort Greely in Alaska. That would have been in breach of the treaty.

The Pentagon suggests that by 2004 that could provide a rudimentary defence in emergencies against long-range ballistic missiles.

And, in an interview with the BBC, the director of the Missile Defence Agency, Lt Gen Ronald Kadish, said the United States would aim to have a limited defence against long-range missiles, and a "robust" defence against short-range weapons in "five to 10 years".

That is viewed with great scepticism in some circles, which argue that even realistic testing is still years away.

They say that the Bush administration has not even worked out what kind of system it wants.

At present there is little evidence one way or the other as to whether the US pursuit of missile defences is encouraging or discouraging weapons proliferation.

The supporters argue that it will deter the development of missile technology, the detractors that it will spur such development.

The key focus of concern is what impact the US programme will have on China, and then India and Pakistan.

While the US system is not really directed at any of them, they have small nuclear missile arsenals that could be affected by missile defences, and are members of a fragile regional security triangle.

Evolution

And how has the Bush administration's thinking on missile defence evolved since 11 September?

Administration officials from President Bush downwards have been talking increasingly about what looks like an emerging "Bush doctrine" of pre-emptive strikes to deal with new threats.

General Kadish agrees that pre-emptive action is "the best kind of missile defence". But he says a defensive system is necessary if missiles are launched against the United States or US forces. Some analysts argue that the main justification for missile defences in future as far as the Bush administration is concerned could be that they will preserve Washington's freedom to intervene in regional hot spots without fear of direct retaliation against its own territory.

The 11 September attacks have in many ways done little to alter the debate over missile defence, except entrench positions on both sides.

The supporters say that the attacks show that the United States must be ready to deal with surprise threats from any direction.

The opponents say missile defences would not have helped on 11 September, and that the Bush administration has got its priorities wrong.