The death of the 100th British soldier in Iraq is no more nor less tragic for family, friends and comrades-in-arms than the 99 deaths of British servicemen that preceded it. Yet, for all that, it is a landmark: a round number that pulls us up short and reminds us anew of the true cost of this war. That Corporal Gordon Pritchard met his death at Umm Qasr, the place where British forces struggled to establish their first bridgehead at the start of the 2003 invasion, added a sad symmetry to the announcement.

It should be acknowledged that the losses sustained by British forces have, so far, been relatively light. The United States has lost more than 2,000 servicemen to date and another US casualty is announced almost daily. Britain has had the advantage of responsibility for the more peaceful southern sector, and ministers have rightly resisted US attempts to deploy British forces elsewhere. The experience in Northern Ireland has also inclined British commanders to favour a less aggressive style of soldiering when engaged in a conflict that involves civilians. In other circumstances, the figure of 100 might have been passed months ago.

That said, 100 British servicemen would not have been in Iraq in the first place if the Prime Minister had not joined President Bush in waging this utterly unnecessary war. With the legality of Britain's involvement still not clarified and public support for the war as low as it has ever been, the resentment of the dead servicemen's families is understandable. Iraq is not a war like any other. It was highly contentious from the start. And it turned out that it had been waged on a false premise: Saddam Hussein had no weapons capable of threatening international security.

While the military campaign was expertly and devastatingly won, the occupation was a catastrophe - for Iraq and the foreign troops whose duty it was to enforce it. As the full scale of the mismanagement becomes apparent, it hardly seems to matter whether, for instance, Paul Bremer was a poor administrator or whether the fault lay with the lack of planning in Washington. The victims were Iraqis who were left with anarchy in place of the liberation they had been promised, and US and British troops who died in the resulting insurgency.

As the months of the Iraq war have turned into years, the Government has successfully played down the true cost of our involvement. Weeks pass with scarcely a mention by ministers of the British troop presence. The impression is created that this is primarily an American war, with American casualties. The news that 100 British servicemen have now died dispels that reassuring illusion. And inevitably it prompts questions about what British troops are doing in Iraq at all, and renewed calls for their withdrawal.

Last month's elections for an Iraqi parliament, which passed off largely peacefully, provide part of an answer. The troops helped create the conditions for Iraqis to cast their votes safely. It is easy to scoff at the inadequacies of Iraq's democracy and ask whether it can survive. But its chances at present may be better with British troops on patrol than they would be without.

This does not mean, however, that the commitment of British troops should be open-ended. The post-Saddam political institutions may be far from perfect, but they are now complete. And while a precipitate British withdrawal would be irresponsible, setting a timetable for withdrawal would not. It would convey to Iraqis that the country will be theirs to govern. And it would reassure servicemen and their families that, while the 100th casualty will not be the last, there is a date by which the troops will be home.