A potent, if in an unexpected way, part of the Iraqi defenses during the Iraq War were the Saddam Fedayeen ("Saddam's Men of Sacrifice"), directed by Saddam's son, Uday Hussein and the operational control of Gen. Iyad Futiyeh Rawi. They were estimated to have 30,000 to 40,000 fighters,  although it was difficult to distinguish between the irregulars of the Fedayeen and Arab, especially Syrian, mercenaries.
It appears to have been started in 1995, used by Uday for personal purposes, smuggling both on behalf of the regime and for private enterprise, and was widely reported to operate a death squad that conducts extra-judicial executions. It was accused of an anti-prostitution campaign that beheaded women. "Many of the victims were not engaged in prostitution, but were targeted for political reasons," according to a March 20 State Department report. They may also act as enforcers of loyalty in regular military units
Uday lost control in 1996, when he tried to transfer heavy weapons, from the Republican Guard, to it. He may have retained control, or simply command.
Equipment and training
They were mostly young men in their teens or twenties, with light infantry weapons such as rifles, machine guns, and had light antitank weapons, such as recoilless rifles and rocket propelled grenades, which could not damage the main armor of a tank but could hurt lighter vehicles, or, with a lucky shot, damage the thinner side or top armor of a tank. Some of their weapons, such as machine guns or recoilless rifles, were mounted on unarmored pickup trucks, called "technicals" after similarly armed vehicles in Somalia. Irregulars also drove explosive-laden civilian vehicles to be used in suicide attacks.
They dressed in civilian clothing. Pentagon officials said on March 24, 2003 that the Fedayeen, who are considered very loyal to the regime, also act as enforcers in regular army units, threatening to kill soldiers who try to surrender. 
Some were first encountered on April 5, 2003, by the first Thunder Run task force. They were usually in civilian clothes"They seemed to have no training, no discipline, no coordinated tactics....nor did the Iraqis seem to understand the lethal and accurate firepower of the M1 Abrams tanks and [[M2 Bradley (armored fighting vehicles)|M2 Bradley infantry firghting vehicles." They would hide in brick bunkers or behind thick trees, and were unaware that not just the armored vehiches' main guns, but their heavy machine guns, easily penetrated their cover.  They would also hide among civilians and, when their fire was returned, noncombatants were killed.
Use by the Iraqis
U.S. forces had expected them to be used in house-to-house fighting in cities. Instead, apparently without regard to casualties, they moved from fruitless attacks on tanks outside the cities, to much more significant attacks on the less well defended resupply convoys. A significant amount of U.S. combat power had to be diverted to protecting those convoys.