The Iraqi government has issued a statement saying that Blackwater USA, which has been doing security operations in support of the United States in Iraq, must end operations and leave Iraq, citing use of excessive force in a recent incident.
The role of independent security firms in military operations has reached the highest level ever under the Bush administration. Often there are reports of "rogue warriors" conducting secret operations that go unnoticed or are "under the radar". Some have said this is the largest privatization of the military in modern history.
To learn a little more about Blackwater USA, you can watch this YouTube video presented by Jeremy Scahill.
At the time of this writing the Blackwater USA website was down. Another good source of information is the Wikipedi Blackwater USA page.