The Mehdi Army militia poses, in the eyes of the Pentagon, the greatest threat to Iraq's security, replacing al-Qaeda in Iraq as the country's "most dangerous accelerant of potentially self-sustaining sectarian violence".
"Safe" cities turned lethal overnight for the coalition in April 2004
To supporters, the militia is the military muscle of Iraq's urban Shia Muslims, fighting to protect Najaf and other Shia parts of the country.
Its membership rose from just a few thousand after the US-led invasion to some 60,000, according to a December 2006 report by the Iraqi Survey Group.
It was created in the summer of 2003, prompted by radical Iraqi cleric Moqtada Sadr, who preached in his sermons the need for a new force.
Young men were recruited at offices near mosques to defend the Shia Muslim faith and their country in defiance of the US-led coalition's arms controls.
In August 2007, Moqtada Sadr announced the militia had declared a ceasefire, under which it pledged not to attack rival armed groups or American forces in Iraq.
The cleric extended the truce in February 2008 but it came under strain amid claims US and Iraqi forces were detaining militia members.
US and Iraqi forces said they were only targeting renegade Mehdi Army factions they accused of flouting the ceasefire.
Access to guns
Taking its name from the Mehdi - a messianic figure in the Shia tradition - the Mehdi Army is fiercely loyal to its religious founder.
Since Iraq's elections and the creation of its own, Shia-dominated government, Moqtada Sadr's movement has continued to take on new members.
It is said to feed on dissatisfaction among Shia who initially welcomed the ousting of Saddam Hussein and the end to curbs on their faith.
The group is also believed to have members across the Iraqi security forces and administration and has been linked to sectarian attacks on Sunni Muslims.
The Mehdi Army's potential as an armed force was first really felt when violence erupted against coalition forces in April 2004 on the streets of Baghdad or Najaf.
The US accuses Iran, Shia Iraqis' spiritual ally, of training, supplying and financing Mehdi Army fighters.
In a November 2006 report on Iraq, the Pentagon described the group as "currently having the greatest negative affect on the security situation in Iraq", and particularly in Baghdad and the southern provinces.
The presence of Mehdi Army members in the Iraqi police force was, the report added, fuelling "Sunni concerns about persecution".
Since the 2004 uprising, the militia's fighters have occasionally clashed with both US and British coalition forces.
Tension in the southern city of Basra reached a new level when the local Mehdi Army leader, Abu Qadir, was killed in May 2007.
And fierce fighting erupted across the city in March 2008 when thousands of Iraqi troops launched a major offensive, overseen by Iraqi Prime Minister Nouri Maliki, against the Mehdi Army and rival Shia militias.
Some US intelligence analysts have questioned whether Moqtada Sadr's control over the group has weakened as the militia has grown.