When the time comes to write the history of America's Iraq war, last Wednesday may go down as the turning point.

That was the day Sunni insurgents blew up one of Shiite Islam's holiest mosques with the clear intention of provoking civil war.

What they also intended was to scuttle U.S. efforts to godfather a government of national unity between the Shiite majority and Sunni minority.

U.S. Ambassador Zalmay Khalilzad knew the last chance for a decent Iraq outcome and a major U.S. troop drawdown lay in persuading Sunnis to embrace politics and stop backing insurgents.

To block Khalilzad's efforts, Sunni jihadis chose a target whose desecration would provoke sectarian slaughter.

Only an extraordinary display of joint leadership by Sunnis and Shiites can prevent a bloodbath. A frightened Iraqi friend told me by phone from Baghdad: "This either brings us together or splits us apart once and for all."

The bombers were diabolically clever in choosing their target. The gold-domed Askariya mosque in Samarra is the 9th-century burial place of the 10th and 11th of 12 imams revered by mainstream Shiites. Shiites believe these imams are the rightful heirs of the Prophet Muhammad.

Shiites also believe the 12th imam, Muhammad al-Mahdi, went into occultation (a suspended state) near the mosque because he feared Sunni persecution. He will supposedly reappear before Judgment Day and rid the Earth of injustice.

Sunni terrorists deliberately struck a shrine that means everything to Shiites and nothing to Sunnis.

In fact, the most fanatic Sunnis consider Shiites to be infidels. Abu Musab al-Zarqawi, the Jordanian terrorist, whose group is linked to al-Qaida, has declared "total war" against Shiites and attacked their markets and mosques.

Al-Zarqawi knows that many Sunnis, who held sway under Saddam Hussein, fear Shiite domination. He also knows that neighboring Sunni Arab regimes, like Saudi Arabia and Jordan, fear the emergence of a "Shiite crescent." They worry that Iran — a country of Shiite Persians — will try to dominate the Arab heartland via its influence with Iraqi Shiites and Shiite minorities in the gulf.

Al-Zarqawi — whom I presume to be behind the bombing — no doubt believed he could destroy the Samarra shrine without much protest from the Sunni Islamic world.

Within Iraq, he aimed to stir up intercommunal violence — scores of Sunni mosques already have been torched by angry Shiites — and overcome the amazing restraint shown until now by Shiite religious leaders after previous attacks.

So we have reached a seminal moment. At a time when Muslims worldwide are protesting cartoons that defamed the Prophet Muhammad, Sunni fanatics have destroyed one of the most sacred Shiite sites. Shiites want to see Sunni Muslims rise up in protest against the bombing.

Even before this outrage, Shiites were angry that Iraqi Sunni leaders had not firmly declared that those who conducted attacks on Shiites were terrorists, not insurgents.

"What do we want from the Sunnis?" a top Iraqi Shiite leader, Humam Hammoudi, declaimed in a recent interview at the Davos World Economic Forum. "We want a common stand against terrorism."

Shiite leaders are waiting impatiently for Sunni leaders to take that stand. But Sunni leaders, angry at Shiite revenge attacks on their mosques, may not be sufficiently forthcoming. The largest Sunni political party has just withdrawn from already stalled negotiations over forming a government of national unity.

Whether U.S. officials can get the two sides together is iffy at this point.

Khalilzad had been working tirelessly to promote a government of national unity before the Samarra bombing. But Shiite leaders were growing angry because they felt he was leaning over too far to entice to Sunnis.

Khalilzad incensed Shiites on Monday when he threatened to cut off U.S. funding for training of Iraqi forces if Iraq's new government failed to appoint nonsectarian figures to run the Interior Ministry.

Sunnis claim that ministry has been infiltrated by Shiite militiamen who kidnap and murder Sunnis. After the Samarra bombing, one top Shiite official castigated Khalilzad for having given a "green light" to Sunni bombers.

Even more disturbing, the top Shiite religious figure, Ayatollah Ali al-Sistani, in a slap at U.S.-trained Iraqi security forces, said, "If the government's security forces cannot provide the necessary protection, the believers will do it."

Until now, al-Sistani has been the most important force holding Shiites back from seeking revenge for previous bombings — or from attacking U.S. soldiers. Now he seems to be encouraging Shiite militias.

This moment is fraught. Either Iraqis leaders rise to the occasion and control their angry followers, or Iraq will slide toward real civil strife, with U.S. troops trapped in the middle. Let us pray.