Posse Comitatus Act of 1879, 18 U.S.C. §1385 (full-text).
The Act is a U.S. federal law passed on June 16, 1878 after the end of Reconstruction. The Act prohibits most members of the federal uniformed services (Army, Air Force, and State National Guard forces when such are called into federal service) from exercising nominally state law enforcement police or peace officer powers that maintain "law and order" on non-federal property (States, their counties and municipal divisions) in the former Confederate states.
The statute generally prohibits federal military personnel and units of the United States National Guard under federal authority from acting in a law enforcement capacity within the United States, except where expressly authorized by the Constitution or Congress. The Coast Guard is exempt from the Act.
The Act provides that:
|“||Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air 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.||”|
Questions regarding which activities violate the Act arise most often in the context of assistance to civilian police. At least in that context, the courts have held that, absent a recognized exception, the Act is violated (1) when civilian law enforcement officials make “direct active use” of military investigators, (2) when the use of the military “pervades the activities” of the civilian officials, or (3) when the military is used so as to subject citizens to the exercise of military power that is “regulatory, prescriptive, or compulsory in nature.” The Act does not apply to the Navy or Marines and does not prohibit activities conducted for a military purpose that incidentally benefit civilian law enforcement bodies.
Application to cybersecurity
Some observers claim that the Act prevents the military from cooperating on cybersecurity with civil agencies that may lack the resident expertise and capabilities of the military and DOD. In addition, it may sometimes be difficult to distinguish a criminal cyberattack from one involving national defense, especially if the attack is on a component of critical infrastructure.
Some have therefore proposed that the Act be amended to clarify when U.S. military can operate domestically regarding cyber threats to such infrastructure, most of which is privately owned. Others maintain that no revision is needed because the President has the authority under current law to direct the military to support civil authorities in the event of a domestic disaster.
A memorandum of agreement signed between DHS and DOD may increase the likelihood that the military would play a significant role in responding to a major cyber attack on U.S. information networks. However, some argue that the defense of U.S. information systems should be solely the purview of civilian agencies such as DHS and the FBI, because involvement of the military creates unacceptable privacy and civil liberties concerns.
- 18 U.S.C. §1385.
- According to DOD doctrine, “direct assistance” by military personnel includes searches and seizures as well as the surveillance of individuals. DODD 5525.5, DoD Cooperation with Civilian Law Enforcement Officials, Encl. 4 § E4.1.3 (Jan. 15, 1986).
- Department of Defense regulations effectively place them under similar constraints, and the limitations on assistance to civilian law enforcement authorities apply to them, 10 U.S.C. §375.
- Pub. L. No. 97-86, 18 U.S.C. §1385.
- See Jeffrey K. Toomer, “A Strategic View of Homeland Security: Relooking the Posse Comitatus Act and DOD’s Role in Homeland Security” (monograph, School of Advanced Military Studies, United States Army Command and General Staff College, Fort Leavenworth, Kansas (July 11, 2002)). 
- Department of Homeland Security and Department of Defense, “Regarding Cybersecurity.” The MOA provides terms for sharing of personnel, equipment, and facilities by the two agencies to improve planning, capabilities, and mission activities in national cybersecurity efforts.
- Federal Laws Relating to Cybersecurity: Discussion of Proposed Revisions, at 10 ("Application to cybersecurity" section).
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