By Patrick Jackson
The Mehdi Army (MA) 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 MA 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.
The MA's appeal is mainly to "those young and desperate Shia in Iraq's urban slums who have not seen any benefit to their lives from liberation", Dr Toby Dodge, an Iraq expert at the University of Warwick, told the BBC News website.
Taking its name from the Mehdi - a messianic figure in the Shia tradition - the militia is fiercely loyal to its religious founder.
"I'm not sure what the aim of the army is or when we will fight, but I will follow Sadr's orders," was how one original volunteer, 29-year-old Kathem Rissan, explained his position to the Financial Times in Baghdad in July 2003.
Since Iraq's elections and the creation of its own, Shia-dominated government, Mr Sadr's movement has continued to take on new members, feeding on dissatisfaction among Shia who initially welcomed the ousting of Saddam Hussein and the end to curbs on their faith.
It is also believed to have members across the new Iraqi security forces and administration and has been linked to sectarian attacks on Sunni Muslims.
Access to guns
The MA's potential as an armed force was first really felt when violence erupted against coalition forces in April 2004, although many of the gunmen in action on the streets of Baghdad or Najaf may not necessarily have been militia members, but ordinary Iraqis defending their neighbourhoods.
The ferocious street battles suggested the MA had access to rocket-propelled grenades as well as heavy machine-guns and the ubiquitous Kalashnikov assault rifle.
As Dr Dodge explains, weapons are widely available in a country where most men would have military training.
After three wars in close succession, Iraq was a highly militarised state at the time of the coalition invasion and arms dumps were left open for months after the old regime fell.
The MA was the first Shia militia to organise on the ground and benefited from a degree of military discipline, making it the natural choice for leading unrest in Shia areas.
The US recently accused Iran, Shia Iraqis' spiritual ally, of training, supplying and financing MA fighters.
In a November 2006 report on Iraq, the Pentagon described the MA as the group "currently having the greatest negative affect on the security situation in Iraq", and particularly in Baghdad and the southern provinces.
The presence of MA members in the Iraqi police force was, the report added, fuelling "Sunni concerns about persecution".
Since the 2004 uprising, MA 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 MA leader, Abu Qadir, was killed on 25 May 2007.
Some US intelligence analysts believe that Mr Sadr's control over the MA has weakened as the militia has grown, with unpredictable consequences.