Andreas Whittam Smith: Here is a way for Britain and the United States to get out of Iraq together

Millions of us correctly forecast that Western occupation would serve to stimulate terrorism rather than to repress it and that civil war might be the final consequence. We warned that British cities might be attacked. That is why we marched against the war.

Once coalition forces were in Iraq, however, many of us thought they should stay and see what good they could do. Help rebuild the country's infrastructure. Assist the oil industry to get on its feet again. Hold the ring while political reform took place. Mostly wishful thinking as it turns out.

Now it's undoubtedly time to make our excuses and leave. But how? Consider what Sir Jeremy Greenstock, the former diplomat who was the Prime Minister's special envoy, said on Saturday. He told the BBC that the UK and US may have to abandon Iraq if central government breaks down and the country is engulfed in chaos. A pullout from Iraq might be needed if the US and UK had no "reasonable prospect of holding it together".

He did not think this had happened yet. But there would be little alternative if Iraq looked as though it were breaking down into a mosaic of different local baronies and militias. "Then, I think, the coalition [would] have to think again about its presence."

Sir Jeremy's comments have a number of interesting aspects. The first is that he even made them. The second is that, by implication, he is ruling out the possibility of Britain leaving unilaterally as several original members of the coalition have already done, Spain not least among them.

I agree with that assumption, for our early departure would make the position of American troops more precarious and their casualties would rise. In any case, we would have shown ourselves to be unreliable allies. Unpleasant and obtuse as President George Bush and his close associates undoubtedly are, we should withdraw only with the Americans and not before them.

Finally, Sir Jeremy describes the conditions that would justify retreat as if they were still hypothetical. This is very diplomatic. I guess that Sir Jeremy thinks that the coalition should leave Iraq soon but doesn't wish to say so in such terms. For the fact is that the central government of Iraq has broken down and the country is engulfed in chaos. It has descended into a patchwork of local baronies and militias. Sir Jeremy's conditions have already been met.

Don't be fooled, however, by the apparent passivity of Mr Bush and Tony Blair. They understand - or at least I think they must do - that the news from Iraq shows them as ever more foolish. And even if they still don't get it, the opinion polls make the same point in their mother tongue. They have spent their entire adult lives calculating what makes them look good, what places them in a bad light. Now they must both fear history will cast them among the failures rather than the heroes. They might even begin to think themselves lucky to escape impeachment for having personally and obstinately led their countries into an unprecedented humiliation. So we can be sure that they are similarly anxious to leave Iraq before it is too late - which is also as soon as possible.

But they won't give as reasons what Sir Jeremy puts forward. His is a diplomat's justification not a politician's. Politicians don't confess failure and then make for the exit. They declare victory before they leave.

Politicians must have a cover story. Central government breaking down and Iraq engulfed in chaos is not a serviceable narrative from their point of view. What they need is a story that fits in with everything that have said since the conflict began. Only when this can be constructed and declaimed from the steps of the White House and 10 Downing Street will American and British troops be brought home.

Luckily, the materials are to hand. The two leaders could write exit speeches based on the new constitution for Iraq. In three week's time, the draft constitution will be put to the Iraqi people for approval. Sensibly, it would create a highly decentralised state. The Federal government would have exclusive powers only over foreign affairs, defence policy, monetary policy and fiscal policy. Everything else would be decided in regions divided among the three communities, Kurds, Shias and Sunnis.

The draft constitution also deals cleverly with the oil question by making a distinction between current production and future exploitation. The oil revenues from current production would be equitably shared between all regions but future oil development would be regionally controlled.

Unfortunately, the proposals fail adequately to guarantee religious freedom and the rights of women, but we can hardly be surprised at such an outcome. All in all, this draft constitution is probably the best that could be done in the circumstances even if it is far from perfect. In sum, the Kurds won't have to be governed closely from Baghdad. The Shias won't have to live in a secular society. The Sunnis won't find themselves completely at the mercy of the Shias or Kurds whom they once ground under foot.

Now isn't this the victory that Mr Bush and Mr Blair require? I believe it is even though the risk remains that the Sunnis reject the constitution by the specified two-thirds majority in three provinces. In that case, fresh elections for a new interim government would have to be held in December followed by a second attempt at drafting an acceptable constitution.

Nonetheless, the draft constitution which exists, likely as it is to be approved by big majorities of Kurds and Shias and most probably by a narrow majority of Sunnis, is a major achievement, and it can be convincingly portrayed as such.

With these arrangements in place, no future Saddam Hussein could make himself dictator. Iran's influence would be confined to the Shia south. What else can the coalition achieve? We shall wait for ever for a well trained Iraqi army to take the place of allied troops. They will never resemble the Grenadier Guards. The Iraqi police service will not be like Scotland Yard or the New York Police Department any time soon. We've done what we can. We must declare victory and leave. Urgently.