IT IS unthinkable, or was so until this week. The possibility that Tony Blair could detach himself from the shoulder of President Bush and walk away from war with Iraq was almost impossible to imagine for the Washington establishment.

But President Chirac and Clare Short have, between them, forced such an outcome on to America’s radar screen. At present, it is only a distant and faint blip. But the veto and resignation threats from the French leader and Britain’s International Development Secretary have placed it there.

For Mr Blair to step back from war now would be a “devastating blow” to Mr Bush, according to David Abshire, former US Ambassador to Nato and a special counselor to President Reagan.

It would wreck US diplomacy surrounding the war, complicate the military campaign, hamper post-war reconstruction and resonate through the rest of Mr Bush’s presidency.

“The success of this effort and the stature of this President and his presidency would be gravely affected if America’s closest ally were to decide against joining an Iraq invasion,” Lee Feinstein, director of strategic policy at the Council of Foreign Relations, said.

Yet it is highly doubtful, indispensable though he has made himself to the United States, that Mr Blair could halt Mr Bush’s march to war by absenting himself. Mr Bush, who unlike the Prime Minister is also Commander-in-Chief of his Armed Forces and who does not have to answer to a parliament, is not as susceptible to pressure from hostile allies and colleagues. He has also written the rules for a showdown in which he cannot afford to blink.

Polls suggest that Mr Bush would be hurt, at least in the short term, by going alone. A slim majority think that the US should wait for allies before taking military action. But that majority is shrinking.

Fifty-five per cent said that the US should go to war even without a UN resolution, to a poll in The New York Times yesterday said, a reflection of the different ways that the United Nations is viewed either side of the Atlantic. It is not just inside the White House that Americans think that the UN unreasonably obstructs US foreign policy interests.

Mr Bush’s “coalition of the willing” would be heavily denuded without Britain on board. Australia is the only other country to have committed troops. But Mr Bush, who since the September 11 attacks likes to cast his presidency in historic terms, would likely still see merit in proceeding alone.

Militarily, the British ground force is now so deeply integrated with the Americans that General Tommy Franks, the US coalition commander, would have to redraft his battle plan and call for more troops and armour if Britain backed out of the campaign.

The British land force of 26,000 troops from 7th Armored Brigade (the Desert Rats), 3 Commando Brigade and 16 Air Assault Brigade has been assigned to enter Iraq as part of the 1st US Marine Expeditionary Force. It is crucial to General Franks’s plan because of the Desert Rats’ heavy concentration of armour and artillery, as well as the versatility of the two lighter-armed brigades.

The Desert Rats have 120 Challenger 2 tanks. Although the 1st US Marine Expeditionary Force has its own embedded Armored division with about 100 M1A2 Abrams tanks, British commanders say that this would not be enough. The removal of 26,000 British troops and their armour from the coalition would be a serious blow to General Franks and his planning staff.

Donald Rumsfeld, the US Defense Secretary, conceded that because of the Prime Minister’s obligations to Parliament, Britain’s precise role remained unclear. “Until we know the resolution, we won’t know what their role will be,” he said.

If Mr Blair failed to fix his political problems with a second resolution, one possibility would be for British troops to step back from the front line, but bear the brunt of peace-keeping, reconstruction and humanitarian work, informed officials said.

Concerns about Mr Blair’s domestic plight are beginning to hit home at the White House. However, there is no sign of any political contingencies being drawn up by the White House to accommodate the absence of Britain. “Sometimes you don’t have a Plan B, and this is the situation the Administration is confronting,” Mr Feinstein said. That is largely based on the White House reading of Mr Blair as a man of honor and a man of his word, the staunchest of all US allies.

Mr Blair is one of the co-architects of the diplomatic campaign against President Saddam Hussein and has provided invaluable cover to Mr Bush against accusations that the White House is acting unilaterally. No gathering in Washington, social or political, in the presence of Britons takes place without praise for the Prime Minister.

Seasoned analysts say that they expect Mr Blair to stay the course. It’s “inconceivable” for him not to, Mr Abshire said. “Blair is a man of history. He has demonstrated an extraordinary profile in courage, and one day there will be a fine statue of him here.”

How they voted

Copyright 2003 Times Newspapers Ltd.

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