But what happens if such efforts, and frantic ones by the United States, prove incapable of stopping an all-out war?

What if, as Abraham Lincoln famously said of the United States' greatest ordeal: "All dreaded it, all sought to avert it. And the war came."

The greatest fear of leaders throughout the Middle East is that an unrestrained civil war, if it ever comes to that, would not only give birth to warring Sunni, Shiite and Kurdish enclaves inside Iraq, but that the violence could also spread unpredictably through the region.

Some experts have advocated a negotiated breakup of Iraq into three main sectors for the main ethnic and religious groupings. But a violent crackup could not easily be kept stable.

It might well incite sectarian conflicts in neighboring countries and, even worse, draw these countries into taking sides in Iraq itself. Iran might side with the Shiites. It is already allied with the biggest Shiite militias, some of whose members seemed to be involved in the retaliatory attacks on Sunnis after the bombing of the Shiite shrine in Samarra last week.

And Sunni countries like Saudi Arabia, Jordan and Kuwait might feel a need to defend Sunnis or perhaps to create buffer states for themselves along Iraqi borders. Turkey might also feel compelled to move in, to protect the Iraqi Turkoman minority against a Kurdish state in the north.

If Iraq were to sink deeper into that kind of conflict, Baghdad and other cities could become caldrons of ethnic cleansing, bringing revenge violence from one region to another. Shiite populations in Lebanon, Kuwait and especially Saudi Arabia, where Shiites happen to live in the oil-rich eastern sector, could easily revolt. Such a regional conflict could take years to exhaust itself, and could force the redrawing of boundaries that themselves are less than 100 years old.

"A civil war in Iraq would be a kind of earthquake affecting the whole Middle East," said Terje Roed-Larsen, the special UN envoy for Lebanon and previously for the Israeli-Palestinian conflict. "It would deepen existing cleavages and create new cleavages in a part of the world that is already extremely fragile and extremely dangerous. I'm not predicting this will happen, but it is a plausible worst-case scenario."

A first question for the United States if a general collapse of order seemed to be in the offing would be what to do with its 130,000 troops in Iraq.

"We would probably have to get out of the way," said Larry Diamond, who advised the U.S. occupation in Baghdad in 2004 and is now a senior fellow at the Hoover Institution at Stanford University in California. "We wouldn't have nearly enough troops to quell the violence at that point. At a minimum, we'd have to pull back to certain military bases and try to keep working the politics."

Modern civil wars have been resolved by negotiations, but only after they were deepened by the intervention of outsiders. Internal conflict in the Democratic Republic of Congo in the late 1990s led to intervention by troops from Rwanda, Uganda, Angola, Zimbabwe and Namibia. The Balkan wars erupted after the breakup of Yugoslavia earlier in the 1990s. The power-sharing arrangements that were worked out for Bosnia and Kosovo remain precarious and backed up by NATO troops.

In events closer to Iraq, more than 15 years of civil war in Lebanon ended when Syrian troops took on the role of reinforcing a peculiar arrangement that distributes certain high offices among Lebanese sectarian groups. Even the West at first welcomed the Syrians as a stabilizing factor - until last year, when Syria withdrew under European and American pressure.

But Iraq poses a threat that dwarfs these problems. The pivot of what could become a regional conflict is almost certainly Iran. Shiite leaders close to Iran won the Iraqi election in December, and although many Iraqi leaders defend their Iraqi nationalist bona fides, a civil war would almost certainly drive them to seek help from Iran.

That could stir Sunni fears of Iranian dominance in the region.

"What you have in Iraq is not just a society coming apart like Yugoslavia or Congo," said Vali Nasr, a professor of national affairs at the Naval Postgraduate School in Monterey, California. "What is at stake is not just Iraq's stability but the balance of power in the region."

Historians looking at such a prospect would see a replay of the Shiite-Sunni divide that has effectively racked the Middle East since the eighth century and extended through the rival Safavid and Ottoman empires in modern Mesopotamia and finally into the Iran-Iraq war of the 1980s. This time, however, suspected Iranian nuclear ambitions could accelerate a nuclear arms race, with Saudi Arabia likely to lead the way among Sunni nations.

While the U.S. secretary of state, Condoleezza Rice, has proclaimed that the world has isolated Iran more than ever because of its nuclear ambitions, Iran has in fact tightened relationships with its local allies as events in Iraq have played out. In recent months, Iran has been deepening its alliance with Syria and the Shiite movement Hezbollah in Lebanon, and it appears ready to strike up a friendship, backed by financing, with a Hamas-led Palestinian Authority.

Some experts, however, say Iran may understand the dangers of a war. Even the denuciation by the Iranian president, Mahmoud Ahmadinejad, of the bombing of the Shiite shrine in Samarra, for which he blamed Zionists rather than Sunnis, could be seen as an act of restraint, these experts say - an effort to play to Shiite anger without fanning flames between Islamic communities in Iraq.

Whatever role Iran plays, many experts see another danger from a civil war in which U.S. forces are forced to the sidelines in angry Sunni areas of Iraq. Those areas would almost certainly become safe havens for terrorist groups posing a long-term threat to other Arab countries and the West, especially the United States and Israel.

"You can be sure that Al Qaeda will set up shop in the Sunni areas, just like they did in the Afghan civil war," said Kenneth Pollack, director of research at the Saban Center at the Brookings Institution in Washington.

Pollack cautions that a civil war could prove especially painful for the Shiites. There is no reason, he says, to assume that they will not fight among themselves. The three major Shiite movements each have militias. Sometimes they have clashed. Iran, he said, would just as soon avoid a violent fragmentation along those lines.

"The first thing you would see in an Iraq civil war is an intra-Shia civil war," Pollack said. "There are a thousand Shiite militias that could do battle against each other, splintering even the southern part of Iraq."

Not all specialists on Iraq think that an eruption of civil war will necessarily draw in outside nations. Turks, for example, might be tempted to intervene, especially if a Kurdish state were set up in the north, tempting Kurds to rebel in eastern Turkey. But Turkey would also not want to alienate members of the European Union, which it is trying desperately to join, according to Morton Abramowitz, a longtime diplomat who is now a senior fellow at The Century Foundation in the United States. "The Turks would not like what's going on, but they're prudent," he said.

Another possible alternative to a huge intervention from the outside could come in the form of an organized regional effort, backed by the United Nations or the European Union, to broker a political solution. Or Sunni Arab states, through an organization like the Arab League, might try to send in an international force to stabilize the country.

Surveying all the nightmare possibilities in an interview late last week, Zalmay Khalilzad, the U.S. ambassador to Iraq, said: "I do not believe that we are heading that way. The leaders of Iraq know that they came to the brink with the attack on the shrine, and there has been an evolution of their attitudes as a result. I simply believe that the leaders of Iraq do not want a civil war."

Lincoln, however, said in retrospect that having leaders who do not want war is not enough. The problem is whether there are things they want more than war, and are willing to accept war to get. In Iraq, it seems, this will also determine whether the leaders will one day say with satisfaction that they stepped back from the brink or, sadly like Lincoln, that "the war came."