Leading article: The spectre of civil war that hangs over Iraq

The stampede on a bridge in the north of Baghdad yesterday made 31 August the single bloodiest day in Iraq since the toppling of Saddam Hussein more than two years ago. The details of how this tragedy unfolded are still hazy, but the hand of Iraq's insurgents is identifiable amid the carnage.

Around a million Shia pilgrims had gathered for the annual commemoration of the martyrdom of Moussa ibn Jaafar al-Qadim, whom they regard as successor to the Prophet Mohamed. Mortar rounds were fired on the Qadhimiya mosque, where Qadim is buried, and later on gathering pilgrims. Then, rumours that suicide bombers were in the crowd instigated mass panic. As people fled, the railings on the bridge leading to the mosque gave way. Hundreds were plunged into the water. Many were trampled. At least 965 died. It is feared the death toll could rise above 1,000. Hospitals struggled to cope with the sheer numbers of the dead and dying. Most of the victims were women and children.

The insurgents may not have deliberately engineered the stampede, but they clearly created the fear that led to it. And the Sunni-led insurgency has shown no compunction in targeting Shia pilgrims in the past. In March last year, suicide bombers killed 180 people in simultaneous attacks on Shia shrines in Baghdad and Karbala. In August 2003, a car bomb exploded outside a mosque in Najaf, killing 85, among them the respected Shia leader Ayatollah Mohammed Bakr al-Hakim. But the scale of the bloodshed makes yesterday different.

So does the timing. Iraq's future is poised on a knife-edge. The long-awaited constitution was presented to the National Assembly at the weekend. But not with the support of the Sunni representatives on the negotiating committee. Sunni leaders are now determined to veto the document in October's referendum. In one sense this can be regarded as an improvement on the Sunni refusal to vote in last year's elections. But a rejection of the constitution would do nothing to promote stability in Iraq. And Sunni resistance to a settlement seen as suited to Iraq's Kurdish and Shia communities is unlikely to be wholly democratic in nature. Iraq must brace itself for more bombings.

What is more, there are reasons to fear that this latest attack has the potential to push Iraq to a new level of violence. Attacks by Sunni insurgents in the Shia heartlands in the south have prompted demands for revenge. But these have been quashed by the Shia leaders in the Supreme Council for Islamic Revolution in Iraq. And the most powerful Shia cleric in the country, the Grand Ayatollah Ali al-Sistani, has been steadfast in countering calls for retribution. This is what has saved Iraq from civil war so far.

But the mortar attack appears to have been aimed at the minority Shia population in the north, who are mostly followers of the young cleric Moqtada al-Sadr. The bonds between Sadr and the Shia leadership are already under strain - last week there were clashes between Sadr's supporters and the Iranian-backed Shia militia of the south. This latest tragedy has the potential to shatter the Shia consensus of restraint.

All this serves to emphasise, yet again, the urgent need for a political settlement in Iraq. Sadr fears an overly federalist constitution just as much as the Sunnis, since it could deprive his northern followers of the benefits of Iraq's oil revenues. Unless a compromise is made between those demanding regional autonomy and those opposed to any form of federalism, Iraq has no obvious route out of the violence. But the sad truth is that each new atrocity - such as occurred yesterday - pushes the prospect of peace even further away.