Imagine you are a US president faced with the decision on whether or not to go ahead with the development of anti-missile defences.

The CIA and other intelligence assessments tell you that long-range missile technology is spreading fast, especially among so-called rogue states who might well be developing chemical or even nuclear warheads with which to arm such weapons.

If - and after this recent test's failure it's a big if - if weapons could be developed to intercept incoming missiles, wouldn't it be a good idea to deploy them?

That is pretty much what President Bill Clinton is thinking. But first he needs to know if a missile defence system is viable. And that's what this three-shot test programme is supposed to decide.

Final chance

Test one - last October - was claimed as a success. But subsequent information indicates that the interceptor hit the target largely by chance after a complicated series of mishaps. The latest test was clearly a failure. And there is only one more to go.

Scientific critics of the programme say that the scale and scope of the testing is just not sufficient to decide things one way or another.

But even if such a system could be deployed, would it be a good idea?

Here too opinion differs. It would require the constraints of the ABM or anti-ballistic missile treaty to be loosened; and if Moscow refused to agree to this, then the United States would have to take the unprecedented step of abandoning the agreement altogether.

With the mood in Washington running very much against arms control treaties and agreements as the sole means of countering proliferation, there is a growing emphasis upon US unilateral action to assure its own defence.

New dis-order

Really what is at issue here are two very different views of the current world order.

Advocates of arms control insist on the continuing viability of agreements like the ABM treaty.

But supporters of missile defences say that the US and its allies - Russia too in their view - face new challenges from countries for whom the complex catechism of deterrence means little.

This new world dis-order requires resolute action. Moscow, they say, knows full well that these defences are not directed at nullifying its deterrent force.

Some US think-tank experts believe that a deal might be reached with Moscow linking changes in the ABM regime to significant further cut-backs in strategic arsenals.

But other analysts fear that the real impact of any defence system will be felt in China; they say Beijing will feel compelled to significantly expand its nuclear capability.

Politics

Missile defence provokes a complex mix of political, diplomatic and strategic questions - especially so in a US Presidential election year.

But with the Republicans strongly in favour of missile defences and Vice-President Al Gore linked to any decision this administration makes, it's going to be a tough call for President Clinton.

But so far it seems impossible to say if missile defences can really be anything other than science fiction.