They are the Army you rarely see: Thousands of heavily armed security contractors protecting top officials and escorting convoys — most paid by U.S. taxpayers.
But unlike American soldiers, critics say these men sometimes operate under vague rules with little accountability.
"It's like the wild, wild West," says Jim Errante, a former contractor in Iraq.
One former manager in Iraq tells NBC News he caught inexperienced, frightened security teams literally shooting civilian cars off the road to clear the way for a convoy.
Though contractors can use lethal force, the U.S. government does not vet who is hired. The Pentagon says it does watch how companies perform and investigates any alleged misconduct.
Retired U.S. Army Gen. Montgomery Meigs — who monitored U.S. contractors in Bosnia — says it's up to companies themselves and to military commanders in Iraq to ensure rules are followed.
"It's difficult. And it takes a special effort by the military chain of command," says Meigs, who also serves as an NBC military analyst. "If they're overburdened fighting a war, it's even more difficult."
Contractors who commit crimes can be prosecuted under U.S. law.
How disciplined are these men?
"[It varies] greatly from highly professional contractors to flat-out dangerous guys," says Col. Thomas Hammes, a Marine instructor at the National Defense University in Washington.
Hammes spent two months working alongside Iraqis. He says some contractors showed outright contempt for civilians. And even good contractors sometimes used tactics that turned Iraqis against the United States.
"If the government is hiring people that are running them off the road and intimidating them, that really undercuts your message," says Hammes.
But contractors say they perform a vital role, with discipline.
"We aren't cowboys," says Chris Taylor, who is a security contractor for a firm named Blackwater. "Nobody's running around with bandanas and knives in our teeth."
Given the dangers in Iraq — where 70 security operators have been killed — they argue that extreme tactics sometimes are necessary.
"When you have people that are ready to kill you if they get the opportunity, it means getting caught in a traffic jam can be a lethal experience," says Doug Brooks, president of the International Peace Operations Association.
Private contractors say they're essential to the war effort, but critics say they sometimes undermine the battle for Iraqi hearts and minds.