How does George W. Bush feel after the July terrorist attacks in London? Perhaps the way Winston Churchill felt after Pearl Harbor. Relief is the wrong word - you cannot feel relief after a bloody attack on a friend. But there is some sense of relief that a friend will be joining you in a struggle. Churchill later wrote in his memoirs that the news made him think that "we had won, after all".

Few in Washington or London today are confident enough to make even such a private declaration of victory. Still, the moment the bombs went off, the UK began responding in ways that seemed bound to bring the two allies closer. What is just as important, however, is something more subtle: a post-bombing Britain is now changing the UK-US alliance.

Consider the UK mood. When the US responded dramatically to the attacks of September 11 2001, some UK citizens thought the Americans were being hysterical. Terror, the IRA had taught, was an old fact of life, a sort of perpetual deathly nuisance that you managed. Part of this reaction was the stoicism of the nation that endured the Blitz. But part was an act of oblivion. What July 7 did was bring many Britons around to the American view that al-Qaeda terror was indeed something new - different certainly from old struggles such as in Northern Ireland.

The second important response involves the policing of terror. British police shot and killed the wrong suspect - a Brazilian whom they mistook for a suicide bomber. This, too, was a terrible event. In the old days it could have triggered a ban on such aggressive policework - armed police in any case being deemed savagely "American". But Tony Blair, UK prime minister, announced that, while everyone regretted the death desperately, "we do believe it is important to give our police every support at this time". Some have called the bitter trade of open civil society for security checks, aggressive police and fortresses "Israelification". Al-Qaeda Israelified the US in 2001. Now it is Israelifying Britain.

There has been a third change - the announcement by Gerry Adams, the leader of Sinn Fein, the IRA's political wing, and the IRA that it is giving up armed conflict. Some will see this as a victory for measured negotiation and an argument against US foreign policy. But it may also represent the IRA's acknowledgement that al-Qaeda bombs have also obliterated whatever patience remained in Britain for the IRA's old methods.

But what about Americans? Overall, the American view is: Britain could have responded to an al-Qaeda terror attack like Spain; instead it responded like Britain. And this in turn is changing the US. One of the worst tendencies in the American personality is to translate tragedy into a tort action. Thus, for example, committees have now spent years examining whether the Twin Towers' collapse was the fault of the engineers who designed them. Americans also try to find a way to blame authorities. In the instance of September 11, for example, there were endless inquiries into the failures of officials from the New York Fire Department to the Oval Office. An inquiry is demanded? Congress and the White House tell themselves there is no harm in giving in.

Not Tony Blair. Asked about formal inquiries into the safety of public transport, Mr Blair rejected them. What is more, his spokesman put his finger on the reason inquiries can be pernicious: they distract from the task at hand, which is bringing down the perpetrators. Mr Blair has his UK constituents; but he also has another, greater, constituency, that of the citizens in the alliance. Some of these "other citizens" are now adopting Mr Blair as their model.

But Mr Blair has increased influence at the highest level as well. For nearly four years, the US has persisted in labelling its war a "war on terror". That label was fine as a stopgap but is proving insufficient over the long term. Terror, after all, is a means, not a cause.

Few politicians have wanted to point this problem out - Mr Bush has avoided it. It seems that Mr Blair may be taking the first steps to clarify what Mr Bush has not: that the war is really a war against militant Islamic extremism.

Some of the most productive work Mr Blair has done in recent weeks has been to push forward, with Spain, the notion of a larger "alliance of civilisations". What is more, he is using the IRA to highlight the special nature of the jihadists' hostility. This week, for example, he went out of his way to note that he did not think "you can compare the political demands of Republicanism with the political demands of this terrorist ideology we are facing now". To win his war, Mr Bush needs to recalibrate it. He already listened intently to Mr Blair; now Mr Blair has the added and terrifying authority of a recent target.

The points here are simple ones. The Bush-Blair alliance is historic, on the scale of Roosevelt-Churchill. The bombings in London have been horrible. But the value of Britain's response is great, strengthening the alliance. Britain has given Americans new hope that this war, if not won, is, at the least, winnable.

© Copyright 2005 Financial Times