The duties of military alliance require that we give the US the troops they need

It's a bad war with a terrible outcome, but shifting troops from the British to US zones doesn't change anything

In debating the request that some British troops be moved from their sphere of operations in southern Iraq to the American zone further north, it is best to begin in the simplest terms. Start by stripping away the political connotations and see it this way.

In debating the request that some British troops be moved from their sphere of operations in southern Iraq to the American zone further north, it is best to begin in the simplest terms. Start by stripping away the political connotations and see it this way.

A hard-pressed friend has asked for help. The instinctive reaction is to say yes and not to count the cost too carefully. That is what friendship involves. I have opposed the war since the beginning and believe it has turned into a colossal disaster lined with deceit and war crimes. But I understand what duty means even in a situation which I deeply regret having come to pass.

Take friendship out of it and substitute the fact that we are allies, members of the same coalition, does that alter the way we should respond to the American request? Not really. Many of the worst moments in the the two World Wars came when the allies quarrelled among themselves. Mutual irritation between the British and French high commands during 1914 to 1918 was destructive. Thousands of lives were needlessly lost as a result.

One might say to a friend in distress - you ask more than I can do, but I will see what help I can give. As it happens, this formulation comes close to reality. What the Americans are requesting is a British battalion, probably the Black Watch regiment, comprising 650 soldiers and equipment. This force is said to comprises the reserve that British commanders hold in Basra to provide reinforcements for dealing with any unusual increase in hostile activity.

Whether this is the case is the first point that MPs need to establish later today when the Defence Secretary, Geoff Hoon, makes a statement in the House of Commons. Is this reserve still needed? This is a question that only British commanders in Basra can answer. They will vividly remember that in the summer British troops came under continuous attack for more than 65 days. This is said to have been the longest period of continuous combat that British forces have experienced since the Korean War more than 50 years ago. These are exactly the circumstances in which a reserve is required. As frontline troops are killed, or injured, or simply become exhausted, fresh forces must quickly fill the gaps. In that way, the likelihood of further British casualties is reduced.

Suppose the conclusion is that the reserve is necessary and cannot be spared. Then the question passes back to London. For the departure of the Black Watch to help the Americans could be made good by dispatching more British troops to Basra. The context, however, remains the same: one over-stretched ally is asking another one for help.

Would this now be a political decision? Hardly. We are discussing a small number of well-trained soldiers and their equipment. Even as an opponent of the war, I am not prepared to argue that such a marginal increment in our contribution to the Iraq coalition requires a full-scale debate in the House of Commons. It would be more alarming if it was discovered that Britain could not meet such a modest request. For that would tell us that out military capability, for which we have paid a high price in terms of national wealth, is very limited. I believe, therefore, that the Black Watch will be sent to assist the American forces.

This brings us to the question what the regiment's terms of engagement should be. For it will find itself in a different situation. It will probably be entering a Sunni area where hostility to the coalition is more strongly felt than it is in the Shia south. So it will be more dangerous. Moreover, the British unit will be directly under American command. Formally speaking, the entire British force in Iraq is under American control, but battalion commanders report to more senior British officers. In the new posting, the commanding officer of the Black Watch would deal directly with the Americans.

There will be one more difference. British troops are taught to be peace keepers. They can mount invasions if they must, but their training is directed to their most likely use - keeping the peace in Northern Ireland and elsewhere. US troops seem to have a different idea; they put down rebellions, violently if necessary. British troops patrol in berets when they can; American troops invariably wear helmets.

Here, then, is the second issue to be raised with Mr Hoon: will any British troops sent to operate alongside American forces require a variation in their existing rules of engagement? Actually this is a rhetorical question for the answer is so obviously "yes". Such new rules would have to be negotiated between three parties, British commanders in Basra, including the Black Watch colonel, the American commanders and British ministers. Rules of engagement do need a political input for eventually public opinion has to support the way in which British troops are employed.

There is a third question to ask Mr Hoon. Why do the Americans require British reinforcements? Three different answers have been given an airing. The first is that the US does not have sufficient forces to mount a successful attack on Fallujah that clears out the so-called "insurgents". The British are being asked to make good a gap that will appear when American troops round the besieged city are reinforced. That is not an entirely convincing response. I argued earlier that Britain could easily find troops to replace the Black Watch in southern Iraq. With even greater facility, so could the Americans fill their depleted ranks.

This means that perhaps a second answer, less precise, needs examination. American officers seem to feel that British troops should share more of the load in Iraq. I think by this they mean more of the casualties. They see that 68 British troops have been killed there compared with 1,062 Americans. They want to remove the perception that this is an American war and that only their soldiers are being killed.

In fact the arithmetic doesn't support the American feeling. In relation to total troops in Iraq, the American mortality rate is 0.0082 per cent and the British almost the same at 0.0075 per cent. The difference is six soldiers. Which means that we have to turn to the idea that underlying this request are the needs of President George Bush's faltering election campaign. It would benefit the President to show the American electorate that they are not alone in Iraq - "look British troops have arrived to march alongside us".

The trouble for us is that we can ponder these suspicions of bad faith as much as we like and prove nothing. They don't provide a case for refusing the American request. We have been asked for help and we should give it. It's a bad war with a terrible outcome, but shifting some hundreds of troops from the British to the American zone doesn't change anything.

I served alongside the Black Watch in Berlin while doing my national service there. In a rather magnificent way, after a night out, they used to commandeer the city buses and drive them back to their barracks. I'd trust them to handle their dangerous new posting well if it is confirmed. And what will Mr Hoon say this afternoon? Just three things over and over again: "I cannot say"; "I do not know"; "local commanders will decide". Magnificent!