Who is Muqtada al-Sadr?
As the standoff in Najaf, Iraq, continues, U.S. officials try to answer that question.
He is charismatic, the son of a revered Shiite cleric killed by Saddam Hussein in 1999.
But at 31, al-Sadr is considered more of a politician than a respected religious leader.
"In terms of the senior clergy, he is just in his diapers. But that is not really what counts. "He comes from the Sadr family and his father was murdered by Saddam Hussein, so he inherits the legitimacy," says Dan Brumberg, a Georgetown University professor and expert on Iraq.
The United States once dismissed him as a thug, but now officials concede that al-Sadr has broad appeal within Iraq's underclass, largely because he's confronted the United States.
What does he want?
The goal of al-Sadr and his followers, the United States believes, is to bring down Iraq's interim government.
"And if they succeed in that then they would be, in their view, positioned to grab quite a large share of power," says Iraq expert Kenneth Katzman of the Congressional Research Service.
But why are Shiite leaders, whom the United States counted on for support, not stopping al-Sadr's rebellion?
U.S. officials say some are afraid to oppose him. Others hope the chaos he's creating will improve their own chances to grab power.
"The very continuation of confrontation weakens the Iraqi government and makes more enemies for the U.S. every single day, not only within Iraq, but outside," says Middle East expert Shibley Telhami.
The Iran factor
There are reports that al-Sadr is getting arms from radical Iranian clerics. The United States says that's possible, but even if true, doubts Iran's government is involved.
If al-Sadr succeeds, what does it mean for the United States? The worst-case scenario — it disrupts Iraq's elections in January, causes civil war and makes it even harder for the United States to get out of Iraq.