Moqtada al-Sadr is a Shi'ite political and militia leader in Iraq, who has been active in Islamic sectarian conflict and internal warfare, but is increasingly involved in the government process. His political organization is called the ; its military wing is variously called the Mahdi Army or the Jaish al-Mahdi.
He was not a significant figure before the fall of Saddam Hussein, although he had relatives that were notable in Shi'a Iraq. The most influential Shiite leader was Grand Ayatollah Ali al-Sistani, who brokered the agreement between SCIRI and the Mahdi Army. 
American civil viceroy L. Paul Bremer, was concerned, in early March, with Muqtada al-Sadr in Najaf, who was becoming increasingly militant in reaching for power as his Shi'ite rival, Sistani, fought for Shi'a power in the constitutional process. He took effective control of the Kufa mosque and preached violence from there.
Complicating the situation with Najaf, [Kufa, and other southern areas were that they were under operational control of the Polish division commander, with subordinate Bulgarian, Central American, Spanish and Ukrainian units, each with different rules of engagement. The Spanish contingent was the one most immediately affected. Bremer asked LTG Ricardo Sanchez to plan a way to deal with the situation there.  Bremer ordered Sanchez on March 27, to close al-Sadr's newspaper, Hawza, for 60 days; this took place without violence but with much protest.
Operations in Najaf
On April 3, special operations forces arrested one of al-Sadr's assistants, and al-Sadr responded with a call for arms. By the 4th, a 1st Cavalry Division platoon was ambushed, unaware that the arrest had taken place or that high vigilance was indicated; Sanchez called this a breakdown in communications at the division commander level; the 1st Cav was taking over from the 1st Armored Division (U.S.).
Al-Sadr's attacks spread across southern Iraq, and he took control of the capitals of Basra, Al Kut, Nasiriyah, and Basra. They fought British and Dutch forces in Basra.  In Kufa, al-Sadr put his base in the Great Mosque, creating the extremely difficult problem of attacking a religious sanctuary in a war where religion was a major factor. 
From the U.S. perspective, he was a threat, but a threat that would not go away. GEN Ray Odierno said, in January 2008, "we are now meeting with them, for the first time. He's clearly moving more to a humanitarian approach." American officials started calling him "the honorable", and GEN David Petraeus, in February, used the honorific "al-Sayyid", referring to him as a descendant of the Prophet. 
- Greg Bruno (16 May 2008), Backgrounder: Muqtada al-Sadr, Council on Foreign Relations
- Sharon Otterman (1 September 2004), Backgrounder: IRAQ: Grand Ayatollah Ali al-Sistani, Council on Foreign Relations
- L. Paul "Jerry" Bremer with Malcolm McDonnell (2006), My Year in Iraq: The Struggle to Build a Future of Hope, Simon & Schuster, ISBN 9780743273893, pp. 122-124
- Patrick Cockburn (11 April 2008), "Warlord: The rise of Muqtada al-Sadr", Independent (U.K.)
- Bremer, My Year in Iraq, p. 312
- Ricardo S. Sanchez with Donald T. Phillips (2008), Wiser in Battle: a Soldier's Story, HarperCollins, ISBN 9780061562426, pp. 335-336
- Ron E. Hassner (Spring 2006), "Fighting Insurgency on Sacred Ground", The Washington Quarterly, p. 155
- Thomas Ricks (2009), THE GAMBLE: General David Petraeus and the American Military Adventure in Iraq, 2006-2008, Penguin, ISBN 987-1594201974, p. 267