It's almost impossible to overstate the importance of last Wednesday morning's destruction of the Askari mosque, a key Shi'a shrine, in Samarra, Iraq. The response, of course, has been swift, violent, and overwhelming. As of late Thursday Iraq time, the Australian Broadcast Corp. placed the national death toll from reprisals at 130, with most of the victims being Sunni. The hardline Sunni Clerical Association of Muslim Scholars claimed that 168 Sunni mosques have been attacked, and some burned to the ground with 10 imams murdered and another 15 kidnapped since the bombing. Despite widespread appeals for calm, the violence continues, fueled by Shiite militias which seem to have been poised for just such a reason for attacking Sunni targets.

There was immediate political fallout, too. In response to the reprisals, on Thursday the Iraqi Accordance Front, the Sunni electoral group that had won 44 parliamentary seats in the Dec. 15 election, angrily pulled out of talks aimed at forming a government of national unity. On the same day, a group of leading Sunni clerics issued a remarkably blunt criticism of their Shiite counterparts, charging that calls for protest in the wake of the bombing had fueled the violence.

In other words, it's a mess. And despite the American media's fixation on an Arab company buying US ports, this is a far, far more important story.

While radical Shiite cleric Moktada al-Sadr (whose militia has mustered in protest) and Iran's hardline cleric leaders blamed the United States and Israel for the attacks, most of the speculation in the Middle East as to responsibility is focusing on Abu Musab al-Zarqawi, the Sunni terrorist leader whose group's attacks on Shiite targets and loose affiliation with al-Qaeda have long made him the leading focus of American "War on Terror" attention. Even before Wednesday's attack, al-Zarqawi's skillful use of the invasion of Iraq as a terrorist recruiting tool, his direct assaults on American and British targets, and his sophisticated use of Iraq as a testing ground for terror techniques have allowed him to easily surpass Osama bin Laden in terms of street credibility in the Muslim world.

If al-Zarqawi is, in fact, the culprit--and it's certainly his style--it remains to be seen how his attack on the shrine, one of Shi'a's holiest sites, will affect that standing. But in the short term, fomenting all-out civil war serves Zarqawi's purposes well. Even if it does not lead to the disintegration of Iraq's fledgling, Shiite-led government, it will (and already has) provoked bloody reprisals that can easily be linked to and used to discredit a government that already stands accused of anti-Sunni torture and death squads. While the majority of Iraqis almost certainly want to avoid all-out civil war, with so much weaponry in the country and such a long history of injustices perpetrated by all sides, this may have been the match that lit the tinderbox.

What can the United States do? Almost nothing. So far, it has invested almost all of its efforts in promoting the legitimacy of the elected, Shiite-led government, even though that government will be closely aligned with Iran, has already perpetrated well-documented abuses, and has essentially imposed repressive Sharia law in the parts of the country (such as the South) where it has firmest control. Washington worked hard to get Sunni parties like the Iraqi Accordance Front to invest in the legitimacy of this political process; it only took 24 hours of ethnic reprisals to destroy those efforts. Sunnis were already leading the anti-American insurgency, and because of our strong political pressure to include them in the government anyway, many Shiites now believe Washington is siding with the Sunnis. (Hence, the wilder accusations that Washington was behind the original bombing.)

The Kurds, also, are seeing their dreams of autonomy under the new constitution threatened. As the inevitable waves of violence and counter-violence wear on, America is left with virtually no friends on any side, and virtually no credibility (other than its sheer military manpower, which it has been reluctant to deploy en masse) as a mediator that can stop the bloodshed.

One of the likeliest outcomes of this attack is an escalation, perhaps a dramatic one, in Iraq's civil war. Another outcome is the likely involvement, finally of the United Nations in Iraqi peacemaking efforts, as the agency is brought in to do the job that Washington plainly cannot effectively do. But don't look for the withdrawal of American troops from Iraq any time soon. President Bush will keep them there due to the civil war and may perhaps even expand their presence, ostensibly to curb the violence--even though we likely will provoke far more violence than we prevent.

But the biggest fallout is likely to be political. Unless Washington and other foreign powers decree otherwise, Iraq--if it stays together as a nation-state, an outcome the United States and the rest of the West are strongly vested in--is almost certainly going to be led in the future by its 60 percent Shiite majority. The question is which Shiites will lead. If the present government fails to form and/or disintegrates, the void is likely to be filled by more radical leaders, particularly clerics like al-Sadr. Al-Sadr has gone in less than three years from being a little-known rebel cleric that occupying US forces identified as a wanted criminal, to being a central power broker among Shiites. Out of this chaos, he, or someone ideologically similar, could well be the one who seizes power. If so, Iraq's transition from a savage secular dictatorship (under Saddam) to a savage cleric-led dictatorship, a transition made possible by George Bush, will be complete.

In the interim, there is an immediate and stark risk of even greater escalation in Iraq's bloodshed, and, as has already been the case, the primary victims will be civilians. More measured leaders on all sides, not to mention all of the international community, would like to avoid this outcome. But it may not be possible. The die has already been cast. In many ways, it was cast at the time of America's invasion, and something like the bombing of the Askari mosque was inevitable at some point.

Nonetheless, the bombing is a critical turning point. This has been an attack which has turned not only a mosque, but George Bush's entire Iraq policy, to rubble.