Analysis
By Andrew Marr BBC News political editor

It is the defining paradox of the politics of the Iraqi crisis, candidly described by Jack Straw on the Today programme this week: if you want to disarm Saddam Hussein, and achieve a peaceful outcome, then you have to be ready to attack him.

Whatever influence he has had, has been bought by his sincerity - his promise to President Bush that almost whatever happens, Britain will be with him

He has agreed to let the inspectors back in, but only because he now believes the threat of military action is sincere.

Yet what Mr Straw did not say is also true: the threat can only be felt in Baghdad as sincere, if it really is.

And there comes a point when that sincerity, that sinew-stiffened, adrenalin-pumping determination, becomes a self-fulfilling prophesy. "We will attack unless...." turns into "We will attack...".

The United States has not reached that point but is self-evidently near to it.

Blair's influence?

President Bush's doctrine of unilateral, strike-first action expressed last week could hardly have been clearer.

His speech to the UN itself carried a sharp warning too. Even as the diplomats struggle to write a resolution that will satisfy European caution and American impatience, the US military is being mobilised.

In all this, we know how Mr Blair's influence has played in only the broadest way.

He had some influence in achieving more UN focus from the Bush administration - we will only know how much when the memoirs are published.

We know he has so far failed over the Middle East peace conference.

We know there is a gap between the US objective of toppling Saddam and Britain's more limited one of removing his weapons of mass destruction.

High-rolling gamble

But we also know that whatever influence he has had, has been bought by his sincerity - his promise to President Bush that almost whatever happens, Britain will be with him.

In private as in public, the Prime Minister lets no chance go by of assuring his listeners that he rates the President, trusts him, and has no intention of walking away.

So it is a high-rolling gamble.

If the UN Security Council agrees a tough, time-limited resolution that results in Saddam allowing the inspectors to find his weapons and destroy them, and the US holds back from war, then Mr Blair will emerge with huge credit, domestically and in the wider world.

If, as a result of all this, Saddam is toppled internally and Iraq emerges from a grim era, the Prime Minister will be wholly vindicated.

Go with Washington?

On the other side, though, are deep perils.

If the Americans lose patience with the UN process as too long-drawn-out, which way does he jump?

If he goes with Washington, the damage to the international system will be severe and he will be hugely unpopular in Europe.

So Mr Blair prefers to debate the Iraqi crisis step by step, and not to talk much about what happens next? One can see why

If that war then went badly, or resulted in large numbers of British deaths, or produced unacceptable civilian casualties in Iraq, or awful terrorist attacks in the UK...

I hardly need to finish the sentence. Labour would split.

Public opinion would be unforgiving. Mr Blair's own position might become impossible.

He knows all this, of course. These are hardly the actions of a focus-group-obsessed, nervously unprincipled leader.

So Mr Blair prefers to debate the Iraqi crisis step by step, and not to talk much about what happens next? One can see why.