Posse Comitatus Act and Related Matters: The Use of the Military to Execute Civilian Law [August 16, 2012] [open pdf - 705KB]
"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. § 1385, in 1878. Congress has also enacted a number of statutes that authorize the use of the 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. […] 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."
CRS Report for Congress, R42659