Martial law is the imposition of military rule by military authorities over designated region(s) on an emergency basis—and often, but not necessarily, only temporarily—when the civilian government or civilian authorities fail to function effectively (e.g., maintain order and security, and provide essential services), or when there are extensive riots or protests, or when the disobedience of the law becomes wide-spread. In most cases, Military forces are deployed to quiet down the crowds, secure government buildings and other key or sensitive locations, and maintain order. Generally, military personnel replace civil authorities and perform some or all of their functions. The constitution could be suspended, and in a full-scale martial law, a high-ranking military officer could take over, or be installed, as a military governor or as the head of the government.
In addition, martial law can be used by governments to enforce their rule on the public. Examples: after a coup d'état (Thailand 2006); when threatened by popular protests (Tiananmen Square protests of 1989); or to suppress the opposition (Poland in 1981). Another example would be The October Crisis which took place in Canada in October 1970. Martial law can be declared in cases of major natural disasters also; however most countries use a different legal construct, such as a "state of emergency".
Martial law has also been imposed during wars or in cases of occupations in the absence of any other civil government. Examples of this form of military rule include Germany and Japan after World War II or the American South during the early stages of Reconstruction. In addition, it is used by governments to enforce their rule on the public. -
Generally, the imposition of martial law accompanies curfews, the suspension of Civil law (legal system), Civil rights, and habeas corpus, and the application or extension of military law to civilians. Civilians defying the martial law could be subject to military tribunal, called a court-martial.