"Initially, we were going to stay until the insurgency was defeated," said James F. Dobbins of Rand Corp., a former special envoy under Presidents Clinton and Bush. "About a year ago, we amended that in a fairly important way by saying we were going to stay until the Iraqi government and its army and police were capable of coping with the insurgency. We redefined victory in terms of the Iraqis' capability instead of the defeat of the insurgency.

"Now even that measure of success has proven elusive," Dobbins said." At this point, I think we would be content if we could diminish our presence, allow the Iraqis to simply hold their own against the insurgency and prevent the country from rupturing into an even more serious civil war than the one that now exists."

The upsurge in violence between Sunni and Shiite Arabs in recent weeks reached a peak after the Feb. 22 bombing of a Shiite shrine in Samarra touched off what one official called "a moment of fear" inside the administration — a sense that events in Iraq could spiral beyond any measure of U.S. control.

In the aftermath of the bombing, U.S. Ambassador Zalmay Khalilzad warned, "We have opened the Pandora's box. . . . There is a concerted effort to provoke civil war."

Rumsfeld, asked whether U.S. forces would intervene in an inter-Iraqi conflict, said, "The plan is to prevent a civil war, and to the extent one were to occur . . . from a security standpoint, have the Iraqi security forces deal with it to the extent they're able to."

Seeking unity

Before the recent violence, U.S. military officials said they hoped to reduce the number of American troops in Iraq from about 130,000 to about 100,000 during the course of 2006. Officials said last week that the violence could slow the U.S. drawdown but added that they still expected some troop reduction to occur.

The good news, Rumsfeld and other officials said, was that U.S.-trained Iraqi security forces did not disintegrate, and Iraqi political leaders, particularly in the Shiite community, quickly intervened to stop the violence from escalating further.

But the senior official warned that the U.S. strategy of nurturing a new "unity government" and building multi-ethnic Iraqi security forces was still dangerously vulnerable to events.

"Sectarian violence . . . is not going down as (quickly) as we would like to see," he said. "A surge further in sectarian violence, way below what I would call a civil war, is still enough to really threaten what we're trying to do there, because it strengthens the militias, it strengthens the radicals, it weakens the security forces."