A one-liner, only half in jest, is doing the rounds in Whitehall: "the road to the euro lies through Baghdad". Numbers 10 and 11 Downing Street agree on one thing: the next two months will determine the Prime Minister's fate. A very successful war will provide him with the political hegemony for which he has long grasped. A very unsuccessful one will finish him. Anything between the two will leave him weakened, but not fatally.
It is within this prism that the great battle between Tony Blair and Gordon Brown is heading for its denouement.
The Blairites' optimistic scenario runs like this: with a war quickly won, with Saddam Hussein found dead or alive, with television pictures showing grateful liberated Iraqis, Tony Blair will assume a new political authority. He would see his risk-taking vindicated, his credibility with voters revived, his brand of interventionism established as a global template.
This shift in the balance of forces would allow Blair, for the first time, to dictate policy to Brown. He could remove him as Chancellor, offering him the job he couldn't accept, that of Foreign Secretary.
The Treasury is monitoring the campaign of attrition. Allies of the Chancellor cite a host of examples. The briefing against his pre-Budget report last November began almost before Brown had sat down (although Treasury officials now admit they woefully underestimated borrowing forecasts). Brown's beloved tax credits for the poor - the centrepiece of his fiscal redistribution by stealth - are disparaged over the road as "a booze-and-fags charter", while responsibility for the impending national insurance rises will be laid at the Chancellor's door right at the time of the Budget on 9 April.
Then there is the battle for public service reform in education and health. The Blairites were furious at Brown's verbal mauling of Charles Clarke during the height of the tuition fees battle, at a pivotal meeting of the cabinet committee on domestic affairs. They have been perfectly happy to see Clarke and Alan Milburn join forces to take on Brown, although they were taken aback by the Health Secretary's willingness to confront the Chancellor.
The Blair-Brown battle in the press has reached another of its cyclical highs. While the Mail is still securely behind Brown, he has spectacularly lost the Sun to Blair. In fact, when Brown went to Wapping on 3 February, ahead of his important speech on the future of markets in public services, he is said to have emerged shaken after a roasting from the paper's new editor, Rebekah Wade, and its political editor, Trevor Kavanagh.
Carefully planted suggestions of Brown's expendability have served a triple purpose: to impress upon the public that Blair Mark II isn't for compromise on any issue; that he can reshuffle any member of his cabinet; and that Brown had better do the right thing on that other historic decision - the euro.
Blair and Brown are yet to have a meaningful discussion about the five economic tests. Brown insists that the "preliminary and technical phase" is yet to be completed, but, inside the Treasury, officials say they've done what they can.
The prospect of war has brought a series of unintended consequences for Blair, most so far detrimental. But it has concentrated his mind on the issue of courage. If (back to the optimistic scenario) he turns around a public opinion so set against war, he will have the confidence to overcome polls on the euro. He will see a 60:40 deficit at the start as a cinch.
There is another reason. Even if the war is easily won, the PM is aware "he's got a hell of a rebuilding job to do" in Europe, says one of his people. He will be very keen to repair links with France and Germany, and to prove Britain's European credentials, which have been deeply damaged. Blair's friend Jose MarIa Aznar, the Spanish prime minister, has told him that, in order to do that, he can't remain outside the euro: "Why, if I can run with both legs, should I run with just one?" Aznar asked him when they met last month.
Encouraged by the strengthening of the euro against sterling, Blair is looking either for a straight "yes" or a "yes, but not quite now" formulation that agrees in principle, but requires certain further "clarifications" before announcing a referendum for 2004. Brown favours a "not quite yet", followed by a "rolling assessment", guaranteeing the continuation of his veto and his chancellorship.
In the meantime, Brown is determined to show that he's behind the war, writing a blank cheque by way of proof.
Iraq has ensured that the euro decision will go to the wire. By mid-May, Brown and Blair will know how the war has gone. They will very quickly draw conclusions about the euro - and their respective fates.
