Provocation in Samarra

This terrorist act was evidently calculated to strike at the core of the belief system of Shi'ite Muslims everywhere. The act expresses a fanatical disdain for key religious concepts that set Shi'ites apart from Sunnis, for the Golden Mosque of Samarra that the early morning explosion destroyed is revered by Shi'ites as the burial place of the 10th and 11th imams -- descendants of the Prophet Mohammed who are honored as the father and grandfather of Imam al-Mahdi, whom Shi'ites call the ''hidden Imam," a messianic figure who is expected to return one day to usher in an epoch of justice on earth.

Sunni Islamist extremists, known as Salafis, commonly regard such Shi'ite beliefs as heresy. Abu Musab al-Zarqawi, the Jordanian commander of the network that calls itself Al Qaeda in Mesopotamia, has made no secret of his intention to kill Shi'ites merely for being Shi'ites and has proclaimed that his strategic objective is to ignite a religious war in Iraq between the two major branches of Islam.

Such a conflict -- with the potential to replicate the bloodletting of Europe's wars of religion between Protestants and Catholics -- would be the ultimate nightmare for Iraqis, their neighbors, and Americans hoping to foster an inclusive, democratic order in Iraq. And the harsh truth is that once Iraq descends into widespread sectarian warfare, American military forces and diplomats will not be able to do much to stop the killing of civilians and the ethnic cleansing that would likely ensue. Such a conflict could produce human suffering worse than what the world witnessed during the Balkan wars of the mid-1990s.

The bombing of the Golden Mosque in Samarra has created a crisis. Fortunately, the most respected senior Shi'ite ayatollah, Ali Sistani, has called for purely peaceful protest, investing his enormous spiritual authority in a call for reason and restraint. Already yesterday, Shi'ites mounted more than 90 attacks on Sunni mosques. What Americans can do is to encourage US-trained police and military units to provide security for both Shi'ite and Sunni Arab communities.

At the same time, America's ambassador to Iraq, Zalmay Khalilzad, needs to redouble his efforts to coax Shi'ite and Sunni Arab political leaders into forming a national unity government able to isolate Islamist radicals from the main body of Iraq's Sunni Arab population. The price of failing to bring Iraq's political factions together could be a catastrophic religious war that lasts for years and spreads across the world's most volatile region.