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. Americans have a tradition, born in England and developed in the early years of our nation, that abhors military involvement in civilian affairs, at least under ordinary circumstances. It finds its most tangible expression in the 19 th century Posse Comitatus Act, 18 U.S.C. Section 1385, which forbids use of the Army and (as amended) the Air Force to execute civil law except where expressly authorized. The exception documents a contrary component of the tradition. Congress has expressly approved the use of the Armed Forces in extraordinary circumstances or where federal manpower to enforce the law was seen as inadequate. Striking the balance between rule and exception has never been easy, but failure to do so has often proven unfortunate. If the rule is too unforgiving, a Shays's Rebellion may go unchecked. If exceptions are too generously granted, a Boston Massacre or Kent State tragedy may follow. The terrorist attacks against the United States in September 2001 produced some calls for more generous exceptions to the rule. The USA PATRIOT Act broadened the permissible circumstances for the use of the military to assist law enforcement agencies in countering terrorism, but Congress also reaffirmed its determination to maintain the principle of the posse comitatus law. The perceived breakdown in civil law and order in Hurricane Katrina's wake evoked more calls to reevaluate the military's role in responding to disasters. The possibility of using military surveillance equipment and resources, including unmanned aerial vehicles (drones), to assist civilian law enforcement has raised some objections based on the military role. 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 Posse Comitatus Act as well as its statutory exceptions, and reviews the consequences of its violation. The Magna Carta provides the first recorded acknowledgment of the origins of the Anglo-American tradition against military involvement in civilian affairs with its declaration that "no free man shall be ... imprisoned ... or in any other way destroyed ... except by the legal judgment of his peers or by the law of the land." Subsequent legislation in the reign of Edward III explained that this precluded punishment by the King except "in due Manner ... or by Process made by Writ ... [or] by Course of the Law," or as later more simply stated, except "by due Process of the Law." Three hundred years after the passage of the Edwardian statutes, Lord Coke and other members of Parliament read these due process and law of the land requirements to include a broad prohibition against the use of martial law in peacetime, an interpretation they compelled King Charles I to acknowledge. King Charles I, preparing for a military expedition in France, had quartered his troops in homes along the southern English coastline. Rioting resulted, and the participants, both military and civilian, were tried and punished by commissioners operating under the authority of martial law. Offended by this peacetime exercise of military judicial authority over civilians, Parliament sought and was granted the Petition of Right of 1628, which outlawed both quartering and martial law commissions. When, in the following century, the British responded to colonial unrest by quartering troops in Boston, the colonists saw it as a breach of this fundamental promise of English law. Their circumstances, however, were not exactly identical to those surrounding the Petition of Right. First, the question arose in the British colonies rather than England itself. England had stationed troops in the colonies to protect them against the French and Indians and had opted for military governorships in other territories. Second, there was no military usurpation of judicial functions. The colonists remained subject to civil rather than military justice, and soldiers who employed more force than civilian law permitted were themselves subject to civilian justice as the trial of the soldiers involved in the Boston Massacre demonstrates. On the other hand, the troops involved in the Boston Massacre were stationed in Massachusetts not for protection against a marauding invader as they had been in the French and Indian Wars, nor to accomplish the transition between civil governments within a conquered territory as they had been after the French lost Canada to the British as a consequence of those conflicts, but as an independent military force quartered among a disgruntled civilian population to police it. Public resentment of the use of the troops in such a manner sparked the incident, which led in turn to further heightened resentment. In any event, the experience was sufficiently vexing that the Declaration of Independence listed among our grievances against Great Britain that the King had "kept among us, in times of peace, Standing Armies without the consent of our legislatures," had "affected to render the Military independent of and superior to the civil power," and had "quarter[ed] large bodies of armed troops among us ... protecting them, by a mock trial, from punishment for any murders which they should commit on the inhabitants of these States." The Articles of Confederation for the newly established United States addressed the threat of military intrusion into civilian affairs by demanding that the Armed Forces assembled during peacetime be no more numerous than absolutely necessary for the common defense; by entrusting control to civil authorities within the states; and by a preference for the farmer in arms as a member of the militia over the standing professional army. The Constitution continued these themes, albeit with greater authority vested in the federal government. It provided that a civilian (the President) should be the Commander in Chief of the Army and Navy of the United States, and civilian authorities (Congress) should be solely empowered to raise and support Armies, provide and maintain a Navy, and make rules for their government and regulation. The Bill of Rights limited the quartering of troops in private homes, and noted that "a well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed." The Constitution, on the other hand, explicitly permitted Congress to provide for calling out the militia to execute the laws, suppress insurrection, and repel invasion. Notwithstanding the founders' aversion to the use of a standing army to control the civilian populace, the Constitution nowhere explicitly prohibits it, and Congress lost no time in authorizing the President to call out the militia for the purposes permitted under the Constitution. Despite the retention of most police powers by the several states, Congress quickly established a law enforcement capability in the federal government in order to effectuate its constitutional powers and provide a means to enforce the process of federal courts. This authority was vested through the President in federal marshals, who were empowered to call upon the posse comitatus to assist them, an authority similar to that enjoyed by the sheriff at common law, and which was understood to include the authority to call for military assistance. Thus, the militia under federal control (and later the Armed Forces ) could operate either as an arm of the government or to support the federal marshal. These two roles of the military were similar in many respects, but, at least in theory, differed in one key aspect: troops serving as a posse comitatus remained subordinate to civil law enforcement authorities, while troops called up to suppress an insurrection or remove an obstruction to the execution of the laws supplanted civil authorities that had been rendered ineffective. In some cases, if the marshal feared he would be unable to control a disturbance even with the aid of a posse, or if local military commanders declined to give assistance, the marshal would try to persuade the President that an insurrection was underway. At other times, the President might order troops to quell what appeared to amount to an insurrection, yet limit the role of the Armed Forces to responding to the requests of the appropriate civil official, possibly dispensing with the need to issue the requisite proclamation under the Insurrection Act. In addition, Congress has from time to time enacted statutes authorizing federal troops to enforce specific proscriptions, sometimes in aid of civil authorities and sometimes (apparently) in their stead, and Presidents have issued proclamations exhorting all federal officials, civilian or military, to assist in arresting a particular conspiracy or uprising. It is not always easy to ascertain which statutory authority (if any) forms the basis for sending in federal troops. Presidents have relied upon the militia and Armed Forces with some frequency for riot control or when in extreme cases they felt it necessary to ensure the execution of federal law. The following sections provide an overview of the domestic employment of military forces and the statutes that govern such use. Soon after Congress was first assembled under the Constitution, it authorized the President to call out the militia, initially to protect the frontier against "hostile incursions of the Indians," and subsequently in cases of invasion, insurrection, or obstruction of the laws. Shortly thereafter, and echoing Article I, Section 8, cl. 15 and Article IV, Section 4 of the Constitution, Congress enacted the Calling Forth Act, authorizing the President to call out the militia in case of invasion or, at the request of a state legislature (or its governor, if the legislature could not be convened) in case of an insurrection within a state.