The Mahdi army

Background: The Mahdi Army was formed in the summer of 2003 by the radical cleric Muqtada al-Sadr shortly after his return from Iran, where he was feted by commanders of the Quds Brigade, which is part of the Iranian Revolutionary Guards. Al-Sadr, the son of a revered Shiite grand ayatollah who had been murdered on the orders of Saddam Hussein, preached in his sermons the need for a force to defend Shiite Muslims from Sunni Arabs.

Membership: The Pentagon estimated the Mahdi Army's manpower in April 2004 to be 3,000 lightly armed militiamen. After the February 2006 bombing by Sunni insurgents of the Al-Askari mosque in Samarra, one of Shia Islam's most revered shrines, the militia's enlistments skyrocketed. Today, the Iraq Study Group estimates the militia's strength at 60,000.

Weaponry: The fighters have access to a seemingly endless supply of rocket-propelled grenades, heavy machine-guns, sophisticated munitions for roadside bombs and the ubiquitous Kalashnikov AK-47 assault rifle. U.S. field commanders say that the Mahdi Army has been armed and trained by Iran. Tehran denies the charge.

History: In April 2004, after a Sadrist news paper was shut down by the U.S. occupation authority for inciting violence, and a key Al-Sadr ally was sought on murder charges, the militia rose up against U.S.-led coalition forces. Fierce fighting took place in the Shiite holy city of Najaf and elsewhere in which the Mahdi Army sustained enormous losses. A truce was declared in June 2004. Al-Sadr apparently sought asylum in Iran, and subsequently ordered his militia to honor a cease-fire.