Blackwater Security who are under contract by the US State Department to operate in Iraq have had their licence cancelled by the Iraqi Government following an incident in which it killed 8 people and wounded 13 others.
There are 20,000 foreign private security workers in Iraq, making them the nation's second largest occupying force.
The first priority for troop withdrawals in Iraq should be for the removal of these private armies. Here is an article which spells out the depth of the problem.
The significance and menace of privatised military operations is spelt out in the book "Corporate Warriors" by P.W. Singer - here is the full book. The matter is also under investigation by the United Nations.