The Posse Comitatus Act and Related Matters:
The Use of the Military to Execute Civilian Law
Updated November 6, 2018
Summary
The Constitution permits Congress to authorize the use of the militia “to execute the Laws of the
Union, suppress Insurrections and repel Invasions.” And it guarantees the states protection against
invasion or usurpation of their “republican form of government,” and, upon the request of the
state legislature, against “domestic violence.” These constitutional provisions are reflected in the
Insurrection Acts, which have been invoked numerous times both before and after passage of the
Posse Comitatus Act, 18 U.S.C. Section 1385, in 1878. Congress has also enacted a number of
statutes that authorize the use of land and naval forces to execute their objective.
The Posse Comitatus Act outlaws the willful use of any part of the Army or Air Force to execute
the law unless expressly authorized by the Constitution or an act of Congress. History supplies
the grist for an argument that the Constitution prohibits military involvement in civilian affairs
subject to only limited alterations by Congress or the President, but the courts do not appear to
have ever accepted the argument unless violation of more explicit constitutional command could
also be shown. The express statutory exceptions include the legislation that allows the President
to use military force to suppress insurrection or to enforce federal authority, 10 U.S.C. Sections
251-255, and laws that permit the Department of Defense to provide federal, state and local
police with information, equipment, and personnel, 10 U.S.C. §§ 271-284.
Case law indicates that “execution of the law” in violation of the Posse Comitatus Act occurs (a)
when the Armed Forces perform tasks assigned to an organ of civil government, or (b) when the
Armed Forces perform tasks assigned to them solely for purposes of civilian government.
Questions concerning the act’s application arise most often in the context of assistance to civilian
police. At least in this context, the courts have held that, absent a recognized exception, the Posse
Comitatus Act is violated when (1) civilian law enforcement officials make “direct active use” of
military investigators; or (2) the use of the military “pervades the activities” of the civilian
officials; or (3) the military is used so as to subject “citizens to the exercise of military power
which was regulatory, prescriptive, or compulsory in nature.” The act is not violated when the
Armed Forces conduct activities for a military purpose.
The language of the act mentions only the Army and the Air Force, but it is applicable to the
Navy and Marines by virtue of administrative action and commands of other laws. The law
enforcement functions of the Coast Guard have been expressly authorized by act of Congress and
consequently cannot be said to be contrary to the act. The act has been applied to the National
Guard when it is in federal service, to civilian employees of the Armed Forces, and to off-duty
military personnel. The act probably only applies within the geographical confines of the United
States, but supplemental provisions of 10 U.S.C. §§ 271-284 appear to apply worldwide.
Finally, the act is a criminal statute under which there has been but a handful of known
prosecutions. Although violations will on rare occasions result in the exclusion of evidence, the
dismissal of criminal charges, or a civil cause of action, as a practical matter compliance is
ordinarily the result of military self-restraint.
This report provides an historical analysis of the use of the Armed Forces to execute domestic law
and of the Posse Comitatus Act, including their apparent theoretical and constitutional
underpinnings. The report then outlines the current application of the act as well as its statutory
exceptions, and reviews the consequences of its violation. This report appears in abridged form as
CRS Report R42669, The Posse Comitatus Act and Related Matters: A Sketch.
https://crsreports.congress.gov/product/pdf/R/R42659