When martial law is in effect, the military commander of an area or country has unlimited authority to make and enforce laws. Martial law is justified when civilian authority has ceased to function, is completely absent, or has become ineffective. Further, martial law suspends all existing laws, as well as civil authority and the ordinary administration of justice. In the United States, martial law may be declared by proclamation of the President or a State governor, but such a formal proclamation is not necessary. Although the U.S. Constitution makes no specific provision for the imposition of martial law, nearly every State has a constitutional provision authorizing the government to impose martial law. The power of martial law, once held to be nearly absolute, has limitations; for example, civilians may not be tried by military tribunals as long as civilian courts are functional. Nonetheless, within the bounds of court decisions, a military commander's authority under martial law is virtually unlimited. Martial law has been declared nine times since World War II and, in five instances, was designed to counter resistance to Federal desegregation decrees in the South. Although a climate of mutual aid has always existed between the military and civilian law enforcement and should continue to exist, Department of Defense personnel are limited in what they can do to enforce civil law. Military personnel cannot be used in surveillance or undercover operations, and they may not be used as informants, investigators, or interrogators unless the investigation is a joint military-civilian operation in which the military has an interest in the case's outcome.