Sectarian Violence in Iraq Limits U.S. Military Response

The violent fallout from a bombing of a revered Shiite mosque in Iraq has made it harder for the U.S. military to carry out its mission and restore order. There are 132,000 U.S. troops in Iraq, but the volatile situation is limiting what they can do. In many areas, the mere presence of Americans would cause anger and invite more attacks.

The United States is in an extremely difficult position -- wanting Iraqi forces to take the lead but also wanting to make sure this violence does not get out of control.

President Bush today praised Iraqi leaders and clerics who have appealed for calm.

"Understand that this bombing is intended to create civil strife, that the act was an evil act," he said.

The U.S. role in this is especially complicated. A senior military official told ABC News that in the coming days, U.S. forces will essentially "sit back and watch" Iraq security forces try to quell any violence, fearing that the U.S. presence would inflame the situation.

"If it's going to be secured, it should be secured by Iraqis," said retired Army Gen. Jack Keane, who is now an ABC News consultant.

Another senior military officer said if the violence escalated, U.S. forces would have to get involved: "I don't see us dropping bombs, but if the Iraqi army collapsed or couldn't handle it, we would have to try to contain the violence."

There are currently 134,000 U.S. forces in Iraq, and the Pentagon hopes that number can be reduced to 100,000 by the end of the year. Asked if escalating civil strife could affect drawdown plans, a senior officer said, "unquestionably."

"People are asking why we have so many forces in Iraq right now, but if this turns bad, 134,000 becomes a very, very tiny number," the officer added.

If more combat troops were needed on short notice, American commanders would rely on the 3,500 men of the 1st Armored Division's 2nd Brigade based in Kuwait.

But they've also discussed another scenario: If a prolonged civil war broke out and the American military was unable to contain it, it could eventually hasten withdrawal.

"If we found ourselves in a protracted civil war that's lasting for months, I think a sense of hopelessness would start to surround the situation," said Keane. "It would probably be an instigator for our withdrawal much faster than anything we've planned."

But for the time being, the United States said the Iraqi forces are performing well. Friday will be key, however, when huge demonstrations are expected. For the first time, Baghdad will be under a daytime curfew as authorities try to keep the situation from spinning further out of control.

ABC News' Martha Raddatz filed this report for "World News Tonight."