According to the Supreme Court, the there is no precise meaning to the term ‘martial law’ . The Constitution does not mention ‘martial law’ at all; and Congress never defined the term. Today, the idea of martial law is considered in the context of an impending doomsday scenario or massive socio-economic crisis.
The most common definition is that martial law is the temporary government set up by a military force after the civil government has been paralyzed, overthrown, or overpowered. Specific examples include invasion, insurrection, or anarchy. In short, military authorities employ martial law to maintain public order and to protect people and property when the civil government collapses.
Generally, martial law supersedes all civil authority during the period in which it is in operation, and it can only exist -- and military power can only be exercised-- over the property of the citizens.
Many people would be surprised, however, to find out that martial law is steeped in American history. Several examples include:
In the states, the power to invoke martial law is given to the governor. Eighteen states provide give the governor authority to declare martial law. Unfortunately, in the last fifty years, state legislatures have given governors more and more power to invoke martial law.
Under state law, for martial law can be invoked by the governor, there must be some condition or trigger. In many states, such conditions are vague. For example, under Missouri law the governor can proclaim martial law “when in [the governor's] opinion the circumstances so warrant."
The U.S. Supreme Court uses the Fourteenth Amendment as a check upon governors' powers in emergency situations. Together with the increasing martial law powers at the state level, the Supreme Court has heightened the scrutiny it traditionally applies in examining instances of state martial law.
During martial law, military authorities can arrest and detain civilians if it appears to be necessary to restore or maintain public order or to protect people or property. Military officers have the authority to not arrest those who are reasonably believed to be engaged in insurrection or disorder; they can detain them without any due process rights.