Each time ministers dream that they have escaped from the Iraq question, it surfaces again and hits them hard on the back of the head. For a couple of days after the bombings in London, it was thought bad taste to mention Iraq. Then, first Charles Kennedy and then the Chatham House think tank broke the unnatural taboo. Now the subject is out of the box and the Prime Minister, despite his efforts this week, cannot put it back in again.

No sane person is making excuses for the London bombers. No one is saying that the al-Qa'ida brand of terrorism started because of the invasion of Iraq. No one is saying we could make ourselves safe by pulling our troops out of Iraq. The point being made is obvious and true, however unwelcome to ministers. The likelihood of young Muslims, whether in Britain or elsewhere, being attracted to terrorism was increased by our action in Iraq.

We attacked a Muslim country on grounds which turned out to be empty. We broke international law. We faced no serious threat from Saddam Hussein and received no authority from the Security Council. We brought about the death of thousands of innocent Iraqis.

The Downing Street spokesman airily dismisses this by saying that nowadays Muslims in Iraq were killing Muslims. Yes, indeed, as a direct result of our invasion and the situation which we created. We removed a cruel and wicked dictator and substituted the scene of carnage and anarchy in parts of Iraq today. We created in Iraq a new base for terrorism, and the world including Britain is less safe because of that.

There is a danger that our ministers will be so busy defending the war they started that their judgement on what needs to be done now will be blurred. There is no case for immediate withdrawal of British and American troops. That would almost certainly make a bad situation worse. The present Iraqi government, though weak and facing increasing charges of corruption, is at least the result of reasonably fair elections.

But it is not enough simply to repeat, as the Prime Minister does, that we will stay until the job is done. By that, he presumably means until we have trained enough Iraqi police and soldiers to protect Iraq against the insurgents. But this is not as simple a policy as it sounds.

Just as it is impossible to wage war without killing innocent people, so it is very difficult to occupy a country without making militants and using force in a way which will do more harm than good. Occupation can sometimes be almost as hard for the occupiers as for the occupied, as we learned long ago in Cyprus and Palestine. Even in the south of Iraq where Saddam's downfall was welcomed most enthusiastically, British troops operate among people for whom foreign occupation is a humiliation. In central cities like Fallujah, the American presence is already part of the problem, not of a solution.

We cannot now take back from the Iraqi government the authority which we have given them. Because oil production is falling way below what was planned, Iraq will continue to need plenty of American and other financial help. But in return, it is reasonable for us to ask the Iraqi government to carry out their duty. The elections which brought them to power were not universal. Most of the Sunni fifth of the population stayed away.

This was a mistake by the Sunnis, but also a disaster for Iraq as a whole, because without Sunnis in government there can be no stability. There has to be a peace process, which means a serious effort to isolate the murderers, many of them foreign, and negotiate with Sunnis who are fed up with violence and anxious to find a way into politics. The peace process in Ireland has never been tidy and still looks messy today. But at least it has stopped the terrorism and enabled Northern Ireland to achieve European standards of prosperity.

The parallel is not exact, but the broad lesson is the same. Our willingness to keep troops for the time being in Iraq should depend on the willingness of the Iraqi government to make that effort of reconciliation. They are trying now to agree a constitution. Good luck to them - but the urgent need is to negotiate an end to the violence, not with the suicide bombers, but with those on whom the bombers rely for support.

Iraq will never be the shining example of capitalist democracy which President Bush imagined in his first term. The attempt to achieve this with British and American tanks and missiles was bound to fail. But if we learn from our mistakes, Iraq could still become a decent Middle Eastern state in which Kurds, Sunni and Shia live together harmoniously and use their country's wealth for the common good.

The author, who was Foreign Secretary, 1989-95, is currently working on a life of Sir Robert Peel