Explainer: What Is the US’s Posse Comitatus Act? Does the US Military Have Arrest Authority?

Loren Voss

The Posse Comitatus Act of 1878 forbids the American military from enforcing laws domestically unless expressly authorized by the Constitution or an act of Congress. Federal troops cannot, therefore, be on the streets enforcing curfews, investigating crimes, or arresting people without an express federal legal justification. 
 

18 U.S.C § 1385: Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army, the Navy, the Marine Corps, the Air Force, or the Space Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.

US law has sought to preserve a clear separation between military power and civilian government. Constitutional and statutory safeguards, such as the Posse Comitatus Act, limit the use of the military to police Americans.

Although the exact scope of “executing the laws” is open to debate, the Department of Defense’s Instruction on Defense Support of Civilian Law Enforcement Agencies provides the military with this list of activities that are prohibited unless a legal exception to the Posse Comitatus Act exists:  

  • interdiction of a vehicle, vessel, aircraft, or other similar activity  
  • search or seizure  
  • arrest or apprehension; stop and frisk; engaging in interviews, interrogations, and canvassing or questioning potential witnesses; or similar activity  
  • force or physical violence, brandishing a weapon, discharging or using a weapon, or threatening to discharge or use a weapon except in self-defense or in defense of other Department of Defense persons in the vicinity or in defense of non-Department of Defense persons, including civilian law enforcement personnel, in the vicinity when directly related to an assigned activity or mission  
  • evidence collection; security functions; crowd and traffic control; and operating, manning, or staffing checkpoints 
  • surveillance or pursuit of individuals, vehicles, items, transactions, or physical locations, or acting as undercover agents, informants, investigators, or interrogators  
  • forensic investigations or other testing of evidence obtained from a suspect for use in a civilian law enforcement investigation in the United States unless there is a Department of Defense nexus or the evidence was obtained by consent  

Are there exceptions to the Posse Comitatus Act? 

There are no express constitutional exceptions, but some statutes allow the military to conduct law enforcement activities. The Insurrection Act is the most well-known of these. It covers three circumstances in which the president can use the military for law enforcement: 

  • during an insurrection against a state when that state requests federal assistance  
  • when unlawful activity or rebellion against the United States makes it impracticable to enforce the laws by court proceedings  
  • when an insurrection or unlawful activity deprives people of a right or protection named in the Constitution, and the state refuses to or cannot protect that right 

Statutes also give the Coast Guard broad law enforcement authority, and other laws allow the military to conduct law enforcement in specific circumstances. These include, for example, protecting national parks or the president, or assisting with crimes involving nuclear material.

Does the Posse Comitatus Act apply to the National Guard? 

The Posse Comitatus Act applies to military forces under the president’s command, which, in most cases, excludes the National Guard. The act’s restrictions do not apply to a state’s National Guard acting under its governor’s authority. In those cases, state law applies. However, if a president federalizes the National Guard (meaning the forces are under presidential command), the Posse Comitatus Act and its restrictions prohibiting the military from conducting law enforcement apply.

Takeaway 

The Posse Comitatus Act is meant to prevent, in most cases, the military from doing police work inside the United States. Exceptions to the act have been rare and generally require an emergency that overwhelms police forces or are for a narrow purpose such as protection of the president. Yet even exceptions to the act do not grant the military unlimited authority. Troops performing policing duties must still respect Americans’ constitutional protections, just as law enforcement agencies must.