Leading article: One by one, the certainties about Iraq are revealed as illusions

This was the week when the already appalling news from Iraq took another sharp turn for the worse. It began with the pictures of British troops fleeing an angry crowd in Basra. It continued with Iraqi ministers admitting that the newly trained police had been infiltrated by militants. And it ended with the governor of Basra withdrawing all co-operation with the British.

Thus evaporated one of the more durable illusions of the whole Iraq mess: that however grievously the Americans might have mishandled Baghdad and their zone of occupation, the south was a happier story. The British were well trained in policing thanks to their experience in Northern Ireland; they knew to wear soft hats and treat the population politely. And anyway the largely Shia south was a different proposition from the fractious Sunni heartland.

Well, maybe it was - but it does not look so different any more. It looks more like one of those classic conflicts from the Empire, when long-forged relationships with local trusties suddenly shattered over a seemingly trivial incident - and regional, ethnic and religious loyalties came rushing back. The question in Iraq's south now is less whether trust can be restored than how real it ever was in the first place.

With the south now also in the grip of unrest, Britain's remaining supporters of the war have less and less to cling to. The situation is quickly surpassing even the most pessimistic of forecasts. When the Conservative leader Michael Howard calls - as he did yesterday - for a thorough reappraisal of the British presence in Iraq, it is clear that some of the stoutest of hearts are faltering. Even Geoff Hoon, who as defence secretary steadfastly kept his counsel, has now admitted that errors were committed, albeit in a self-serving and roundabout way.

As American casualties mount, and US television shows the gathering mayhem in and around Baghdad, the mood in the United States is also changing. This week the proportion of Americans opposing the war exceeded 50 per cent; in this nation of optimists, a bare 20 per cent of those polled thought that the US would win the war. After a summer in which opponents of the war have been emboldened by Cindy Sheehan's protest outside President Bush's ranch, record numbers are likely to attend today's rally in Washington.

We hope and expect that today's march in London will be similarly well attended. With current US and British policy towards Iraq so patently not working, this is a moment when the voices of protest not only need to be heard, but also have the potential to make a difference.

Politically, neither Mr Bush nor Mr Blair can summarily reverse tracks. Nor, however desirable, is there the faintest chance that Mr Blair would withdraw British troops without agreeing such a step with the Americans. The invasion of Iraq was an enterprise of joint foolhardiness. It is, however, high time to make a sober assessment of what, if anything, has been achieved and what, if anything, can realistically be attained in the future, with a view to setting a timetable for an orderly withdrawal.

There was one sliver of good news from Iraq yesterday. Grand Ayatollah Ali al-Sistani, the moderate Shia leader, has called for a yes vote in next month's constitutional referendum. Once, such an appeal would have been greeted as offering hope of progress towards representative government. But we have been this way before. Elections have done nothing to turn back the tide of violence. It is a measure of how much the mood has changed that the ayatollah's words now sound like so much whistling in the dark.