By Martin Lumb
BBC News Online
Saddam Hussein's army may, on paper, be one of the biggest in the world but it is a small group of irregulars - the Fedayeen - which has been providing much of the resistance to the advancing Americans.
Saddam's son, Qusay (r), is believed to head the Fedayeen
Intensely loyal to Saddam Hussein, the war has given it the opportunity to rise far above the role of ruthless political enforcer which it originally held.
American commanders say the Fedayeen has stiffened the resolve of regular army commanders through intimidation, refusing to allow them to surrender, and providing the manpower for harassment and sniping on enemy columns.
Western intelligence believes the Fedayeen to number between 30,000 to 40,000 young people.
It began in Saddam Hussein's home region of Tikrit and recruited only from loyalist areas.
THE LOYAL FEDAYEEN
Founded by Saddam Hussein's son Uday in 1996
Up to 40,000 members
Drawn from areas loyal to Saddam Hussein
Answerable only to the president's family
It reports directly to the presidential palace, rather than through the army command.
Before being drafted into the fight against the US-led invasion, it was responsible for patrol of borders and controlling - or facilitating - smuggling.
The Fedayeen was founded by Saddam Hussein's eldest son Uday in 1995, but command is now believed to rest with his second son, Qusay.
Qusay is presumed to be heir apparent and has been put in charge of defending Baghdad, as well as Tikrit.
From the beginning, Fedayeen units consisted to a large extent of local toughs who could be relied upon to protect the president and his family, put down dissent and carry out much of the police's dirty work.
It is believed to include a special unit known as the death squadron which, according to dissident sources, carries out executions of opponents of the regime, often in the victims' own homes.
The Fedayeen operate completely outside the law, above and outside political and legal structures.
Night curfews, common in many regions of Iraq, are generally enforced by the Fedayeen.
As fears of an American attack grew in recent times, policing of the curfew was intensified.
In Baghdad, the Fedayeen control main intersections and block entrances to major thoroughfares and sensitive areas.