The Posse Comitatus Act was originally enacted in 1878, in a largely successful effort to prevent the use of federal troops for enforcing the Fourteenth Amendment and other Reconstruction-era measures such as the Civil Rights Acts of 1866 and 1875 and the Ku Klux Klan Act of 1871. All this was in order to facilitate the re-establishment of white supremacy in the former Confederacy. Fearing precisely that, President Rutherford B. Hayes opposed the measure, but it was passed over his veto. Despite these unsavory origins, it has become firmly embedded in American culture.
The United States is unique in that, by law, the military cannot act as police inside the U.S. In all domestic waters the American Armed Forces are bound by the Posse Comitatus Act, which imposes limits on the use of federal military forces in domestic law enforcement.
Posse Comitatus does not actually restrict all military support to law enforcement. Under certain circumstances such support is permitted, as outlined in Army Regulation 500-51, "Emergency Employment of Army and Other Resources, Support to Civilian Law Enforcement, 1 July 1983."
· Actions taken for furthering a military or foreign affairs function of the United States, regardless of incidental benefits to civilian authorities.
· Actions taken under the inherent right of the U.S. Government to insure preservation of public order, and carrying out of governmental operations within its territorial limits, by force if necessary.
Naturally, there are detailed restrictions on the role of the military even in these situations, to preserve the separation between civil and military authority. In addition, Congress has empowered the Armed Forces to support anti-drug operations in the Caribbean and Gulf of Mexico. A series of legislative initiatives during the ‘80s modified Title 10 of the US Code to make the Department of Defense (DoD) the lead agency in international waters for the detection and monitoring of air and sea traffic likely to be involved in the illegal international trade in drugs, with the authority to pass the information to law enforcement agencies, including the U.S. Coast Guard. In addition, the Department of Defense can provide equipment and facilities to law enforcement agencies, and provide training and expert advice. Thus, although uniformed DoD personnel cannot directly participate in searches, seizures, arrests, or similar activities, they can provide important support to law enforcement agencies (e.g., federal troops cannot arrest someone for a civil violation, but can guard someone who has been arrested by a suitably authorize civilian law enforcement agency). The legislation further mandates that the appropriate departments arrange for Coast Guard Law Enforcement Detachments (LEDETS) to be aboard "every appropriate surface navy vessel at sea in a drug interdiction area," outside U.S. territorial waters.
So while the armed forces can perform certain roles in law enforcement, for the most part these are strictly support roles. Despite this restriction, these arrangements have worked pretty well. But new law enforcement missions have arisen in the aftermath of the terrorist attacks on the United States on September 11, 2001.
In a curious loophole, Posse Comitatus does not restrict the Navy from conducting essentially law enforcement operations against persons suspected of engaging such acts as piracy, drug smuggling, the slave trade, and other activities defined by international agreements as "universal crimes." That is, provided those individuals are neither American citizens nor in American territorial waters. Thus, in January 2002, Navy personnel boarded and searched the Hajji Rahmeh, a Syrian merchant vessel, in the Mediterranean Sea. Had the ship been in US coastal waters, or under US registry, however, however, the Navy would be prohibited from taking such action or even directly supporting a Coast Guard boarding team.
Oddly, as written, Posse Comitatus does not apply to the Navy or the Marine Corps. Naval forces repeatedly supported law enforcement authorities in the late nineteenth and early twentieth centuries. For example, naval personnel supported the Coast Guard in "anti-rum running" operations during Prohibition, and Marines were used repeatedly to suppress riots in cities near naval bases, as well as to provide security details for the U.S. Mails in 1921 and 1926, and suppress the Alcatraz Prison Riot of 1946. This changed with the establishment of the Department of Defense in 1947. An administrative ruling by legal eagles in DoD decreed that the act now applied to the Navy and Marine Corps.
. . . the Posse Comitatus Act (reference (v)) . . . is applicable to the Department of the Navy and the Marine Corps as a matter of DoD policy. . .
And in practical terms, even the Army has from time to time engaged in actions that were legally in violation of Posse Comitatus. For example, in the aftermath of the San Francisco earthquake of 1906, despite having no authorization from Washington (with which he was unable to communicate in any case), Maj. Gen. Frederick Funston declared martial law and put troops on the streets.
There have been recent calls to repeal or modify Posse Comitatus. However, this is likely to be very difficult to carry off. The Posse Comitatus Act has become deeply embedded in civil libertarian thought in America, on both the left and the right. For example, the very limited use of military reconnaissance aircraft to support the apprehension of the "D.C. Sniper" during October of 2002 led to a good deal of acrimony. A the time the ACLU protested the measure, warning of a potential "slippery slope" in the erosion of constitutional rights. Similar protests have been heard from the right, the Cato Institute, for example, warning of "grave consequences" that might attend the modification of the law.