Mr Blair must avoid the impression that he is bolstering Mr Bush's election hopes

More than 18 months have passed since the Prime Minister was forced into making a choice that he had hoped against hope he could avoid. He had to choose between following the United States into war in Iraq without a UN resolution or joining France, Germany and Russia in calling for more time for UN inspections. The master of triangulation could suddenly find no third way. He chose loyalty to Washington and threw in his lot with President Bush. It was a fateful decision.

More than 18 months have passed since the Prime Minister was forced into making a choice that he had hoped against hope he could avoid. He had to choose between following the United States into war in Iraq without a UN resolution or joining France, Germany and Russia in calling for more time for UN inspections. The master of triangulation could suddenly find no third way. He chose loyalty to Washington and threw in his lot with President Bush. It was a fateful decision.

Now, Mr Blair is forced to confront another choice that has no third way and must be equally agonising, for it is essentially a new test of that same loyalty. The Ministry of Defence confirmed yesterday it had received a request for assistance from the Americans. It is believed that this would entail the transfer of a British army battalion from the relative safety of southern Iraq to an area much closer to Baghdad, where it would be dependent on US supply lines and, it is assumed, come directly under US command. The redeployment would allow an equivalent number of US troops to join the assault on Fallujah.

This request - not the first of its kind, but the first to become public and the first, it appears, to which Britain is likely to agree - lays bare an uncomfortable truth. Like it or not, British forces in Iraq are part of a coalition that is commanded and dominated by Americans. If, on operational grounds, the US calculates that more troops are needed in a particular place and the British are best placed to supply them, Mr Blair may have little alternative but to agree. If the request eventually comes in the name also of Iraq's interim prime minister, Iyad Allawi, who could argue that until Fallujah is pacified there can be no prospect of elections, it will be doubly difficult for Mr Blair to refuse.

The awkward reality of the "multinational" coalition and its command structure has been concealed until now by the fact that British forces are operating in a zone of their own, with their own supply lines, which has been generally more peaceful than the areas occupied by the Americans. British casualties have been a fraction of those suffered by the Americans to the point where it has almost been possible to forget that our two countries are fighting the same war. If British units are moved to a different area, with different - US - rules of engagement, that could rapidly change. This, in turn, could have a knock-on effect on the south, turning the mood dangerously more hostile than it has hitherto been. Both eventualities make the requested transfer of British troops a high political risk for Mr Blair.

Still more perilous for Mr Blair is the timing of the request so soon before the US presidential election. Hence the procession of Blairite loyalists on to the airwaves yesterday to deny any connection. John Kerry has successfully linked Mr Bush's perceived unilateralism with US problems in Iraq to the point where the President's lack of allies has become a liability. In Iraq, Mr Kerry tells his rallies, the US is bearing 90 per cent of the cost and 90 per cent of the casualties. What more effective way for the Bush campaign to curb that argument than to have more British troops more dangerously deployed, visibly relieving Americans of the burden?

Perhaps the intention was that news of the deployment would not emerge until it was a done deal. Perhaps there really is no connection with the US election and it really is all about the planned elections in Iraq. Appearances, after all, can be deceptive - but they are also, as the Prime Minister knows almost better than anyone, the very stuff of politics. And the old convention - that governments do not seek to influence other countries' democratic elections - is still the best one. Mr Blair may calculate that his domestic standing over Iraq is so low that he has nothing at all to lose. But he must absolutely not create the impression that he would place British troops at risk to help get Mr Bush re-elected. There should be no question of new British deployments until the US election is well and truly over.