There has to be a state of emergancy to declare martial law.
In strict dictionary terms, martial law is the suspension of civil authority and the imposition of military authority. When we say a region or country is "under martial law," we mean to say that the military is in control of the area, that it acts as the police, as the courts, as the legislature. The degree of control might vary - a nation may have a civilian legislature but have the courts administered by the military. Or the legislature and courts may operate under civilian control with a military ruler. In each case, martial law is in effect, even if it is not called "martial law."
Martial law should not be confused with military justice. In the United States, for example, each branch of the military has its own judicial structures in place. Members of the service are under the control of military law, and in some cases civilians working for or with the military may be subject to military law. But this is the normal course of business in the military. Martial law is the exception to the rule. In the United States, the military courts were created by the Congress, and cases can be appealed out of the military system to the Supreme Court in many cases. In addition, a civilian court can petition the military for habeas corpus.
Article 1, Section 9 states, "The privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it." Habeas corpus is a concept of law, in which a person may not be held by the government without a valid reason for being held. A writ of habeas corpus can be issued by a court upon a government agency (such as a police force or the military). Such a writ compels the agency to produce the individual to the court, and to convince the court that the person is being reasonably held. The suspension of habeas corpus allows an agency to hold a person without a charge. Suspension of habeas corpus is often equated with martial law.
Because of this connection of the two concepts, it is often argued that only Congress can declare martial law, because Congress alone is granted the power to suspend the writ. The President, however, is commander-in-chief of the military, and it has been argued that the President can take it upon himself to declare martial law. In these times, Congress may decide not to act, effectively accepting martial law by failing to stop it; Congress may agree to the declaration, putting the official stamp of approval on the declaration; or it can reject the President's imposition of martial law, which could set up a power struggle between the Congress and the Executive that only the Judiciary would be able to resolve.
In the United States, there is precedent for martial law. Several times in the course of our history, martial law of varying degrees has been declared. The most obvious and often-cited example was when President Lincoln declared martial law during the Civil War. This instance provides us with most of the rules for martial law that we would use today, should the need arise.
The President can declare martial law when circumstances warrant it: When the civil authority cannot operate, then martial law is not only constitutional, but would be necessary: "If, in foreign invasion or civil war, the courts are actually closed, and it is impossible to administer criminal justice according to law, then, on the theatre of active military operations, where war really prevails, there is a necessity to furnish a substitute for the civil authority, thus overthrown, to preserve the safety of the army and society; and as no power is left but the military, it is allowed to govern by martial rule until the laws can have their free course. As necessity creates the rule, so it limits its duration; for, if this government is continued after the courts are reinstated, it is a gross usurpation of power. Martial rule can never exist where the courts are open, and in the proper and unobstructed exercise of their jurisdiction. It is also confined to the locality of actual war."
On 8/26/2005, in the wake of Hurricane Katrina, New Orleans was placed under martial law after widespread flooding rendered civil authority ineffective. The state of Louisiana does not have an actual legal construct called "martial law," but instead something quite like it: a state of public health emergency. The state of emergency allowed the governor can suspend laws, order evacuations, and limit the sales of items such as alcohol and firearms. The governor's order limited the state of emergency, to end on 9/25/2005, "unless terminated sooner."