§ 1385, is perhaps the most tangible expression of an American tradition, born in England and developed in the early years of our nation, that rebels against military involvement in civilian affairs. Congress in some cases specifically authorized the use of the armed forces to execute civilian laws in cases that did not amount to insurrection, including enforcement of embargoes and the neutrality laws. When the Act Does Not Apply

		Constitutional Exceptions

The Posse Comitatus Act does not apply "in cases and under circumstances expressly authorized by the Constitution," 18 U.S.C. Statutory Exceptions—Generally

The Posse Comitatus Act does not apply where Congress has expressly authorized use of the military to execute the law. It may not be used in any way that could undermine the military capability of the United States; the civilian beneficiaries of military aid must pay for the assistance; and, under § 275, the Secretary of Defense must issue regulations to ensure that the authority of §§ 271 to 284 does not result in use of the armed forces to make arrests or conduct searches and seizures solely for the benefit of civilian law enforcement. Willfully Execute the Laws

The act is limited to "willful" misuse of the Army or Air Force. In this context, the tests used by most contemporary courts to determine whether military forces have been used improperly as police forces in violation of the Posse Comitatus Act were developed out of disturbances in 1973 at Wounded Knee on the Pine Ridge Indian Reservation in South Dakota and inquire whether: (1) civilian law enforcement officials made a direct active use of military investigators to execute the law; (2) the use of the military pervaded the activities of the civilian officials; or (3) the military was used so as to subject citizens to the exercise of military power which was regulatory, prescriptive, or compulsory in nature. Military Coverage

		Navy and Marines

The Posse Comitatus Act proscribes use of the Army or the Air Force to execute the law. There seems every reason to consider the National Guard part of the Army or Air Force, for purposes of the Posse Comitatus Act, when in federal service. Consequences of Violation

		Prosecution

The Posse Comitatus Act is a criminal statute under which there has apparently been only a couple of prosecutions. Compliance

The most significant impact of the Posse Comitatus Act is attributable to compliance by the armed forces. As administrative adoption of the act for the Navy and Marines demonstrates, the military has a long-standing practice of avoiding involvement in civilian affairs which it believes are contrary to the act, and which date back to military acceptance of civilian authority since the founding of the Republic.
The Posse Comitatus Act states 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." 18 U.S.C. § 1385. It reflects an American tradition that bridles at military involvement in civilian affairs. Congress, however, has approved a number of instances where extraordinary circumstances warrant a departure from the general rule, particularly in cases where the armed forces provide civilian assistance without becoming directly involved in civilian law enforcement. However, a number of statutes, including the Insurrection Act, permit the military directly to execute federal law or provide assistance to states in the throes of insurrection where state officials are unable to execute the law.
Three tests have developed to determine when the Posse Comitatus Act is violated. Courts examine whether: (1) civilian law enforcement officials made a direct active use of military investigators to execute the law; (2) the use of the military pervaded the activities of the civilian officials; or (3) the military was used so as to subject citizens to the exercise of military power which was regulatory, prescriptive, or compulsory in nature. The Act has apparently been used for only a couple of prosecutions, and it is more frequently invoked collaterally as a usually unsuccessful means to avoid jurisdiction or exclude evidence acquired with the assistance of the military. Less frequently, plaintiffs have sought relief in the form of civil damages for an alleged violation.
The Posse Comitatus Act does not apply where the Constitution expressly authorizes the use of the armed forces to execute the laws in such a role. Although the Constitution does not seem to provide such an express exception, military doctrine permits military commanders to exercise emergency authority and to take action to protect federal property.
Although the Posse Comitatus Act applies on its terms only to the Army and Air Force, Department of Defense regulations governing military assistance to civil authorities limit the roles of all of the armed services, including civilian personnel, to prevent their direct involvement in law enforcement activities. The National Guard is covered by the act only when acting in federal service.
The most significant impact of the Posse Comitatus Act is attributable to compliance by the armed forces. As administrative adoption of the act for the Navy and Marines demonstrates, the military has a long-standing practice of avoiding involvement in civilian affairs which it believes are contrary to the Act.
This is an abridged version of CRS Report R42659, The Posse Comitatus Act and Related Matters: The Use of the Military to Execute Civilian Law, in which the authorities for the statements made here may be found.