No single official in the United States has a clean, statutory power to declare martial law. The president cannot do it unilaterally, because no federal statute grants that authority and the Constitution splits military power between the executive and Congress. Most state governors can declare martial law under their state constitutions, but only within their own borders and usually only for a limited time before the legislature has to weigh in. Whoever declares it, courts remain open, and the Supreme Court has repeatedly held that a declaration does not by itself let the military take over civilian courts or civilian life.
Across all of U.S. history, researchers have counted 68 separate declarations of martial law, nearly all of them at the state or territorial level.
Why the President Cannot Declare Martial Law Alone
The Constitution never uses the phrase “martial law.” The president is Commander-in-Chief of the armed forces, but Congress holds the power to raise armies, declare war, and regulate how the military operates on American soil. Congress has passed detailed laws governing domestic military deployment, and none of them authorize the president to impose martial law on his own.1Constitution Annotated. ArtII.S2.C1.1.14 Martial Law Generally
The Supreme Court reinforced this in its analysis of the Milligan case, observing that both Congress’s war powers and the president’s command authority “are derived from the Constitution, but neither is defined by that instrument.” Only Congress, the Court said, can authorize the substitution of military tribunals for civilian courts, and even then only in wartime.2Legal Information Institute. Imposing Martial Law
The Posse Comitatus Act adds a criminal backstop. It makes it a federal offense to use the Army, Navy, Marine Corps, Air Force, or Space Force for civilian law enforcement unless Congress has specifically authorized it. Violators face up to two years in prison. Because martial law by definition involves the military enforcing civilian law, a presidential declaration without a congressional exception would collide with this statute immediately.3Office of the Law Revision Counsel. 18 U.S.C. 1385 – Use of Army, Navy, Marine Corps, Air Force, and Space Force as Posse Comitatus
The Insurrection Act Is the Closest Federal Tool
The Insurrection Act is the main statutory exception to Posse Comitatus and the closest thing federal law offers to martial-law authority. Invoking it is not the same as declaring martial law, but it does let the president put troops on American streets in ways that overlap with martial law’s practical effects.
The Act works through three provisions:
- Under 10 U.S.C. §251, the president may send federal troops into a state at the request of the state legislature or governor when an insurrection breaks out against the state government.
- Under 10 U.S.C. §252, the president may deploy the military unilaterally when rebellion or unlawful obstruction makes it impossible to enforce federal law through the ordinary courts.4Office of the Law Revision Counsel. 10 USC 252 – Use of Militia and Armed Forces to Enforce Federal Authority
- Under 10 U.S.C. §253, the president may intervene with military force when a state fails or refuses to protect its residents’ constitutional rights.5Department of Defense. 10 U.S.C. 331-335 – Insurrection Act
Before deploying troops, the president must issue a proclamation ordering the insurgents to disperse and go home within a set time. This step is required, not optional.6GovInfo. 10 U.S.C. 334 – Proclamation to Disperse
The Act has been used many times, most famously by President Eisenhower in 1957 to send the 101st Airborne Division to Little Rock and by President Kennedy during the desegregation of the Universities of Mississippi and Alabama. What the Act does not do is authorize the president to shut down civilian courts or replace them with military tribunals. Deploying troops under the Insurrection Act leaves the ordinary court system intact.
Governors and State Declarations
Governors have far clearer authority to declare martial law than the president does. Most state constitutions explicitly grant emergency military powers to the governor, and state-level declarations account for nearly all of the 68 in U.S. history. A governor’s authority stops at the state line.
When a governor activates the National Guard for a state mission, those troops serve in “State Active Duty” status, operating under state command and state law. In that status, they are not subject to the Posse Comitatus Act and can perform law-enforcement functions that federalized troops cannot. The picture flips when the president federalizes the Guard under Title 10: those personnel then fall under federal command, carry out federal missions, and are bound by the same restrictions as the regular military.
Most states cap how long an emergency declaration can run without legislative sign-off. Some require legislative approval after 30 days, others allow 45 or 60, and a few use shorter windows. The common thread is that emergency powers are not meant to become permanent, and the legislature has to weigh in eventually.
How Courts Keep a Declaration in Check
Whoever declares martial law, the courts do not close, and detainees can still challenge their confinement in federal court. Two Supreme Court cases set the ceiling on what any declaration can actually do.
Ex Parte Milligan (1866)
Lambdin Milligan, a civilian in Indiana during the Civil War, was arrested, tried by a military tribunal, and sentenced to death. Indiana was not a war zone, and its civilian courts were open. The Supreme Court overturned his conviction and established a rule that still governs: military tribunals cannot try civilians where civilian courts are open and functioning.7Justia U.S. Supreme Court Center. Ex Parte Milligan, 71 U.S. 2 (1866)
Justice Davis wrote for the majority: “Martial rule can never exist where the courts are open and in the proper and unobstructed exercise of their jurisdiction.” Even when habeas corpus has been suspended, the opinion added, a civilian with no military connection cannot be tried, convicted, or sentenced anywhere other than an ordinary court of law.
Duncan v. Kahanamoku (1946)
After Pearl Harbor, Hawaii’s territorial governor declared martial law with President Roosevelt’s approval. The military ran the islands for nearly three years and used military tribunals to try ordinary criminal cases against civilians. The Supreme Court struck this down. Even though the Hawaiian Organic Act explicitly authorized the governor to impose martial law, the Court held that the statute was never meant to let the military supplant civilian courts.8Justia U.S. Supreme Court Center. Duncan v. Kahanamoku, 327 U.S. 304 (1946)
Congress, the Court said, “had in mind, and did not wish to exceed, the boundaries between military and civilian power, in which our people have always believed.” The ruling reinforced Milligan and added a practical rule of statutory reading: when a law appears to authorize martial law, courts will construe that authority as narrowly as possible.
Habeas Corpus and Who Can Suspend It
Habeas corpus is what makes judicial review of martial law work. It lets anyone in custody ask a court whether the detention is lawful. Without it, the military could hold people indefinitely with no oversight.
The Constitution allows the writ to be suspended, but narrowly. Article I, 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.”9Legal Information Institute. Clause II – Suspension Clause and Writ of Habeas Corpus
Two features of that clause matter. It sits in Article I, which governs Congress, not Article II, which governs the president. Most constitutional scholars read this to mean that only Congress can suspend the writ. Lincoln suspended it unilaterally during the Civil War, Chief Justice Taney ruled that action unconstitutional, Lincoln disregarded the ruling, and Congress later passed legislation retroactively authorizing the suspension.
Even during a valid suspension, the Milligan Court clarified, the writ still issues. A court receiving a habeas petition will examine whether the suspension itself was constitutional and whether the particular detainee falls within its terms. Suspension limits the remedy; it does not make detention unreviewable.10Constitution Annotated. Suspension Clause and Writ of Habeas Corpus
What Changes for Civilians Under Martial Law
When martial law takes effect, military authorities take over functions normally handled by police and local government. Typical consequences include curfews, travel restrictions in and out of the affected area, and bans on public gatherings. Military forces become the primary enforcers of order, and their authority overrides local police.
The court system is where the legal change is sharpest. Military tribunals may be set up to handle cases that would ordinarily go to civilian courts. They operate under very different rules: juries are military officers appointed by the military rather than citizens drawn at random, conviction requires a two-thirds vote instead of unanimity, evidence standards are looser (anything a “reasonable person” would find relevant, rather than the federal rules of evidence), and appeals go to a review panel appointed by the Secretary of Defense rather than to a civilian appellate court.
Under Milligan and Duncan, this substitution is constitutional only in the narrowest circumstances: active combat zones where civilian courts literally cannot function. Where courts remain open, military tribunals have no jurisdiction over civilians no matter what a declaration says.7Justia U.S. Supreme Court Center. Ex Parte Milligan, 71 U.S. 2 (1866)
First Amendment rights are also at risk. Press censorship, speech restrictions, and interference with newsgathering have all occurred during past declarations. Courts have consistently held that the First Amendment does not disappear under military authority.
How a Declaration Ends
There is no single, clearly defined federal process for terminating a martial law declaration. The law in this area is, as legal scholars have described it, “complicated and unsettled.” In practice, martial law ends when the declaring authority lifts it, when a court rules it unconstitutional, or when a legislature acts to end it. The Milligan Court offered a guiding principle: “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.”1Constitution Annotated. ArtII.S2.C1.1.14 Martial Law Generally
State-level oversight is more structured. Most states require the governor to obtain legislative approval to extend an emergency declaration beyond a set number of days, ranging from as few as 10 in some states to 60 or more in others. If the legislature declines to extend it, the declaration expires. Federal courts also retain the power to review any declaration and to order the release of detained individuals when the declaration is unconstitutional or no longer justified by the circumstances that produced it.