History throws up uncomfortable symmetries. The foreign policy pursued by Tony Blair was originally forged in the aftermath of a failed adventure in the Middle East more than four decades ago. The principal actors in the drama were Britain and France on one side, the US on the other. The prime minister's policy is now being tested to destruction by another military foray in the region. This time the divide is between the US and Britain, and France.

Mr Blair's decision to go to war against Iraq conflated Gladstonian moralism with conventional strategic purpose. It was right to rid the world of Saddam Hussein; and, yes, he did believe the Iraqi leader had weapons of mass destruction. Above all, though, regime change in Baghdad would mark the limits of the west's tolerance of unconventional weapons proliferation.

Behind such judgments also lay a strategic lesson drawn from Suez, a humiliation at the hands of Washington that had rudely exposed post-imperial Britain to the realities of American power. Henceforth, Harold Macmillan decreed, warm ties with the US would be deemed vital to the nation's security, but they would be balanced and buttressed by engagement in Europe.

Mr Blair did not expect his support for George W. Bush to rupture friendships in Europe. On the contrary, he thought he could keep Americans and Europeans together and thus confirm Britain's role as a pivotal power. But the transatlantic bridge, in the unfortunate metaphor, collapsed under the weight of French and German opposition to the Iraq invasion. The consequent acrimony, visible again at last week's European Union summit, forced Mr Blair to make the choice he had hoped to avoid.

To be fair, the bitterness has also robbed Jacques Chirac of serious options. The French president knows well that a credible European foreign policy demands accord between its two activist powers. That explains why, even after the Iraq war, he invited Mr Blair to join France and Germany in a new triumvirate. The idea has been lost to mutual mistrust. Last week Mr Blair's place at the trilateral table was taken by Spain's José Luis Zapatero. Yet the geopolitical reality is as it was. Britain and France hold Europe's seats on the United Nations security council; if they are at odds, Europe cannot speak with one voice.

There is a still deeper crack, though, in the foundations of the Anglo-American relationship. The traditional balancing strategy is predicated on US support for a cohesive Europe: Britain can lever its position in Washington into influence in Paris or Berlin (and vice versa) only if the US looks kindly on European integration. The re-election of Mr Bush challenges that basic assumption.

The US president's preference for ad hoc coalitions over fixed alliances favours a strategy of divide and rule. In the absence of the Soviet threat, the administration questions whether it has a national interest in a more united Europe. Hard-line conservatives see greater advantage in dividing the continent into "old" and "new".

Mr Blair cannot admit this. The reality of the military commitment in Iraq binds him to Mr Bush. So too does his - mistaken - certainty that the transatlantic bridge can be rebuilt much as before. Mr Bush will laud his guest at the White House this week by delivering a long-requested statement of intent to revive the Middle East peace process. Yet Mr Blair's hand is weakened nonetheless by estrangement from France and Germany.

National interest has always trumped sentiment in the US view of the special relationship. As Stephen Wall, until recently Mr Blair's European adviser, pointed out in a telling lecture at Chatham House on Monday, US presidents have regularly brushed aside the preoccupations of British prime ministers. I had forgotten how John F. Kennedy almost casually wrecked Britain's nuclear planning by cancelling the Skybolt missile system; and just how hard the US state department fought to preserve US neutrality over the Falklands. (France, of course, was then the faithful ally.)

None of this is to say that Mr Blair's government could, or should, simply switch its affections to Europe. A knee-jerk response would defy political and strategic realities. There is too much tied up in the relationship, including such politically radioactive issues as the future of the country's nuclear deterrent, for Old Labour tantrums. Nor are Britain's long-term security interests served by transatlantic animosities.

Yet Britain cannot indefinitely ignore the profound strategic shift in Washington. At some point, just as happened after Suez, a British government will have to look afresh at the nation's place in the world.It could do worse than conclude that, for all that Britain is, and should remain, a staunch ally of the US, it is actually part of Europe.

philip.stephens@ft.com

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