Bush and Blair in strategic agreement, but the political pressures are starting to tug them apart

Donald Rumsfeld could have deflected the question with a smile or a glower, as he does so many others. Instead, the master of the Pentagon turned oddly pensive at the regular briefing on Tuesday.

His answer was careful, measured and nuanced. Yes, Britain might possibly take a smaller role than planned in the looming war against Iraq. Indeed it might take no part at all. Britain was in a "distinctive situation". The words struck like a thunderbolt. Was the global diplomatic meltdown forcing a decisive parting of the ways between America and its most faithful ally?

The short answer is, no. President Bush and Tony Blair are in full strategic agreement over the need to disarm Saddam Hussein. This is no rift at the top, nor is it the end of "the special relationship".

But the warmest diplomatic ties, the closest military co-operation and the most trusting sharing of intelligence cannot overcome the massive and differing pressures tugging Britain and America in different directions.

In many respects, they are working as closely together as ever, maybe more so than ever in the military and intelligence fields. Indeed, that collaboration, and so much shared training, makes the very thought of sitting out the hot part of an Iraq war so galling to British commanders.

The closeness was visible too, at a more personal level, in the atmosphere at the farewell dinner at the British embassy a few weeks ago for the departing ambassador, Chris Meyer, most unusually hosted by the Foreign Secretary himself.

Washington's great and good – cabinet members, supreme court judges, the Chairman of the Joint Chiefs, even Karl Rove, the reputed Svengali behind Mr Bush – turned out in rare number. Behind the good cheer, you sensed gratitude too. In a treacherous world, these luminaries were among foreign friends they could trust.

But military collaboration, the intimate sharing of intelligence, the warmest diplomatic occasions cannot mask objective and differing realities.

To start with the most basic one. America is the lone superpower and global cop; Britain is an upper-middling power torn between Atlanticism and Europeanism. Mr Blair wants to have it both ways but, a few weeks ago, he left no doubt where, if push came to shove, his priority lay.

The first principle of British foreign policy, he told Britain's assembled ambassadors in London, was that "we should remain the closest ally of the US and, as allies, influence them to continue broadening their agenda". The reverse, however, is simply not true. The US may value British friendship but it has a vast array of other fish to fry.

Another different reality is 11 September and its aftermath. Mr Blair may have rushed to the US to watch from the VIP gallery as Mr Bush addressed a special session of Congress after the attacks. But they did not happen in Britain. Britain's attitude to the world was not transformed like that of America.

And then there is public opinion. Mr Bush still benefits from the rallying effect of 11 September; amazingly, he has convinced roughly half his fellow citizens that Saddam Hussein had a hand in the attacks without a shred of serious evidence to that effect. Mr Blair could never pull off that sort of conjuring trick. That is one reason 55 per cent of Americans support an invasion of Iraq even without a second United Nations resolution, and only 19 per cent of Britons do.

Mr Blair was alive to that risk early on. Together with Colin Powell, he persuaded a reluctant President last summer to go the UN route, when the Iraq hawks like Mr Rumsfeld and Dick Cheney, the vice-President, felt that would cause only trouble and delay. At the start of this year, as British public opinion turned even more hostile to war, it was again Mr Blair who desperately (and ultimately successfully) lobbied Mr Bush to go back to the UN for a second resolution which both London and Washington insist is technically unnecessary.

By the time Mr Blair came to Washington on 31 January, the White House understood the size of the domestic political risk he was running. "His neck is on the line," one senior US official, who had sat on the two hours of talks, said immediately afterwards. Maybe now the US will do him one more favour, by embracing and then forcing through the amended UN resolution Britain is now finalising – slightly delayed deadline, benchmarks and all.

If so there will be one final outing before war for the "good cop, bad cop" routine which Washington and London are perfecting. Mr Bush and Condoleezza Rice, his national security adviser, drum their fingers on the desks in impatience but agree to go along. Then they get on the phone to bribe or bully the waverers.

By contrast Britain, for whom the stakes are far higher, projects a more accomodating image, as its diplomats work the coalface in New York. Even Sir Jeremy Greenstock, our urbane, supremely professional man at the UN, says he's "busting a gut" to round up the elusive votes. These were "the tensest times of my career," he confessed the other day – and the exhaustion etched on his face shows it.

The American juggernaut, and the loyal British shoveller on the road ahead of it, may well achieve their goal of the nine vote "moral majority" on the Security Council that might make Mr Blair's position marginally easier. The fact is though that this policymaking-on-the-hoof, arm-twisting version of diplomacy might not have been needed if the US had employed a politer, more sensitive, and more traditional version of diplomacy far earlier in the process.

America may have helped Blair in these last stages. Until then however its behaviour had been anything but "helpful" – to use the word of which the Foreign Office is so fond. Mr Blair has had above all to fight against a quite unprecedented tide of anti-Americanism, which has grown ever since Mr Bush took power.