The U.S. Army has 37,500 military police (MPs), which are, as the name implies, used to perform police functions. That includes patrolling and crowd control. The active army, a pretty disciplined force, doesn't need that many, so 22,000 of the MPs are in reserve and National Guard units (which many civilian policemen belong to.) In wartime, MPs are also used to take care of prisoners of war and keeping the peace among civilians in captured territory. Some 8,000 MPs have been sent to Iraq, and several thousand more are on the way. Because of their usefulness in peacekeeping, the Department of Defense is increasing the MP force by 3,600 in the next few years.