By Andrew Marr The BBC's political editor

Tony Blair now faces the endgame of a long, frustrating and complex diplomatic struggle that has swamped him for months.

He may be a cautious player at home, but he has been gambling that, when it comes to the Iraqi crisis, he can prevent the West falling apart.

To do that means two things.

It means persuading an angry and rampantly powerful Washington administration to swallow its impatience and stay inside the United Nations process of inspections, report-backs and resolutions before any war begins.

Blair's gamble has more than just his reputation at stake

But with every day the military build-up in the Gulf challenges that timescale: is the real momentum in the corridors of the UN in New York, or among the sandy transient camp of American tanks, aircraft and infantry taking shape in Kuwait, south of the Iraqi border?

And his gamble has a second element. He also needs to persuade the other big European players on the UN Security Council, France and now Germany too, that there is a real and unavoidable need for war if Saddam does not back down.

So far, he has had little visible success, though British diplomats still hope for a last-minute gallic shrug, and agreement if Iraq fails to comply.

Click here to send your questions for Andrew Marr to answer in the Panorama Interactive programme, Tackling Saddam, this Sunday

Of course, Russia and China matter too.

But if the Germans and French fail to accept a second resolution authorising war, then Iraq will have produced the worst rift in Atlantic relations since the Suez crisis... and Tony Blair faces his unenviable worst-possible choice, of going with a US-led war, or sticking with his European allies and standing to one side.

Garnering support

In truth, he seems to have already made that decision.

After the language of support and menace used by the prime minister and his Foreign Secretary Jack Straw in recent days, the idea of Britain not backing America seems unthinkable.

It would certainly shock President Bush to the heels on his Texan boots.

If the prime minister gets it wrong, it could destroy his reputation

Mr Blair has been working hard to help create a "coalition of the willing" who would help attack Iraq, even without a second UN resolution.

The British position is already that Saddam is in breach of Resolution 1441 by failing to help the inspectors more actively - even though public opinion in Britain does not yet see that as justifiable grounds for war.

Lonely choice

If the US attacks without explicit further UN backing, and/or the French and Germans fail to give them political cover, then the prime minister enters dark and uncertain territory.

A short, relatively bloodless removal of the Iraqi dictator might actually bolster his position, at least at home.

But a bloody and difficult war would stoke up deep hostility inside the Labour Party, among the electorate, and across Europe too.

This is a sober and probably rather lonely moment for the prime minister.

If he gets it wrong, it could destroy his reputation.

Then again, if he gets it wrong it could destroy rather a lot of other people too.

Panorama: Tackling Saddam was broadcast on Sunday, 2 February 2003, at 22:15 GMT on BBC One and on the Panorama website.