David Runciman: 9/11 marked the beginning of the end - for Brown

The roots of Labour's present crisis lie in the wreckage of 9/11. Decisiveness is key to a leadership coup: Brown has just sat on his hands

In the speech he delivered to the Labour Party conference in the immediate aftermath of 9/11, Tony Blair told his audience in a memorable phrase that "the kaleidoscope has been shaken, the pieces are now in flux". But as we wait for his final speech five years on, we know that nothing could have been further from his own mind. Whatever was in flux in his thoughts before September 11 2001, the moment the planes hit the buildings, everything became fixed and certain. His convictions about the world have been utterly rigid from that point on: this was a war to the death, America's enemies were the enemies of civilisation, and Britain had an absolute obligation to follow America's lead in how they chose to fight back.

The chaos in the Labour Party over this past week has its roots in that moment five years ago when Blair's world changed, but his worldview didn't change at all. Rather, it became set in stone. Ever since, Britain's foreign policy has not been something that was up for debate, or open to reasoned argument. It has been a question of conviction and absolute faith, to be tested to destruction.

Most of the analogies that are being drawn between Blair's current predicament and the demise of Margaret Thatcher are therefore the wrong ones. Almost the only thing people in the Labour Party are able to agree on at the moment is that they shouldn't repeat the Tories' mistake in the way they bundled their leader out of office, opening up wounds that poisoned the party for a generation. But this ignores the fact that the coup against Thatcher was remarkably successful. It freed the Tory Party from a leader whose world view had become totally inflexible, and it served its immediate purpose by enabling them to win an unprecedented fourth term in office.

What did for the Tories was Black Wednesday, and that was a legacy not of Thatcher's removal from office but of her having been allowed to remain there too long. John Major's government was destroyed by the disastrous misjudgements of Margaret Thatcher's third term, which stemmed from her fixation with Europe, and the way that this fixation allowed her authority to leak away.

This is the scenario Labour now finds itself in. Like Thatcher's third tem, Blair's has been marked by an increased fixation on his central political conviction - that Britain must support the US whatever the costs - but an increasing inability to urge this view on either his party or the wider public. As his authority wanes, so his convictions have started to sound more shrill and his judgement has become more erratic, as we saw this summer in the lurching confusion of his Lebanon policy.

In Thatcher's case, this mix of inflexibility and impotence led to the decisive error, foisted on her by her Chancellor, John Major, of joining the Exchange Rate Mechanism at the wrong rate - a fatal combination of weakness and hubris. For Blair it has meant sticking to his post 9/11 guns - following the Americans back into Afghanistan in an effort to clean up the mess they left there first time round - but fatally hampered by the lack of either verbal or financial support from his Chancellor, Gordon Brown. In another combination of weakness and hubris, the British army has been left overstretched and underresourced, and Blair's own position has become thoroughly exposed.

It is not hard to imagine a scenario in which Brown reaps the consequences of all this, just as Major did. If Brown succeeds Blair, he will inherit an unworkable foreign policy in which he himself is deeply implicated, just as Major inherited an unworkable economic policy for which he was in large part to blame.

Some future catastrophe for the British army - some Black day on which their lack of adequate resources leads to a rout and an ignominious retreat - may enable Prime Minister Brown to extricate himself from the mess he allowed his predecessor to get into. But it will almost certainly come at a terminal cost to his own reputation and that of his party.

Even if there is no military calamity, it is hard to see how Brown can escape from the legacy Blair will bequeath him. Britain's place in the world is now inextricably bound up with the hollow rhetoric of Blair's open-ended and unyielding commitment to the "war on terror". It might have been different if Blair had been forced from office in the summer of 2004, when the ruthlessness of such an act could have been deployed to mark a clean break with what had gone before.

The Tories, too, might have saved themselves a decade of grief if they had had the guts to depose Thatcher in the autumn of 1989, when she was first challenged by Anthony Meyer for the leadership, and before she was able to wreak the damage of her final year in office.

It is true that electorally successful Prime Ministers are hard to get rid of, and their supporters will always describe the attempt as an undemocratic coup. Yet that doesn't mean that the correct lesson to draw from the Thatcher years is that it is a mistake to try. It simply means that decisiveness is essential, and half-heartedness to be avoided at all costs.

But Brown has sat on his hands for too long and allowed the poison of Blair's obsessive faith in Bush's wars to enter Labour's bloodstream. Blair's final conference speech, like all those since 2001, will contain its ritual observance of the litany of horrors that terrorism threatens for Western civilisation. It will also pay tribute, on the anniversary of 9/11, to all those who died on that awful day, and all those who have died since defending freedom.

What Blair will not say is that the British soldiers who continue to die are fighting a battle that has now been lost - the battle to prove to people that Blair was right all along, and that his instinctive reaction to the destruction of the Twin Towers was the correct one. As Brown listens, in his usual impassive, introverted way, he may reflect that 9/11 didn't just mark the beginning of the end of Blair's premiership, but the beginning of the end of his own premiership as well.

Dr David Runciman is the author of 'The Politics of Good Intentions'