A military prison is a prison operated by a military. Military prisons are used variously to house prisoners of war, unlawful combatants, those whose freedom is deemed a national security risk by the military or national authorities, and members of the military found guilty of a serious crime. There are two types: penal and confinement-oriented, where captured enemy combatants are confined for military reasons until hostilities cease [citation needed]. Most militaries have some sort of military police unit operating at the divisional level or below to perform many of the same functions as civilian police, from traffic-control to the arrest of violent offenders and the supervision of detainees and prisoners of war.