"They may be called upon to help with civil unrest and crowd control..."
Posse Comitatus Act
"The Posse Comitatus Act is a United States federal law (18 U.S.C. 1385) passed on June 16, 1878 after the end of Reconstruction. The Act prohibits most members of the federal uniformed services (the Army, Air Force, and State National Guard forces when such are called into federal service) from exercising nominally state law enforcement police or peace officer powers that maintain "law and order" on non-federal property (states, their counties and municipal divisions) in the former Confederate states."
The statute generally prohibits federal military personnel and units of the United States National Guard under federal authority from acting in a law enforcement capacity within the United States, except where expressly authorized by the Constitution or Congress. The Coast Guard is exempt from the Posse Comitatus Act."
The Posse Comitatus Act and the Insurrection Act substantially limit the powers of the federal government to use the military for law enforcement."
Source:
en.wikipedia.org
But...
"The original 1878 Posse Comitatus Act was indeed passed with the intent of removing the Army from domestic law enforcement. Posse comitatus means "the power of the county," reflecting the inherent power of the old West county sheriff to call upon a posse of able-bodied men to supplement law enforcement assets and thereby maintain the peace."
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"While the act appears to prohibit active participation in law enforcement by the military, the reality in application has become quite different. The act is a statutory creation, not a constitutional prohibition. Accordingly, the act can and has been repeatedly circumvented by subsequent legislation. Since 1980, Congress and the president have significantly eroded the prohibitions of the act in order to meet a variety of law enforcement challenges."
One of the most controversial uses of the military during the past 20 years has been to involve the Navy and Air Force in the "war on drugs." Recognizing the inability of civilian law enforcement agencies to interdict the smuggling of drugs into the United States by air and sea, the Reagan Administration directed the Department of Defense to use naval and air assets to reach out beyond the borders of the United States to preempt drug smuggling. This use of the military in antidrug law enforcement was approved by Congress in 10 U.S.C., sections 371381. This same legislation permitted the use of military forces in other traditionally civilian areas of immigration control and tariff enforcement."
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"Is the Posse Comitatus Act totally without meaning today? No, it remains a deterrent to prevent the unauthorized deployment of troops at the local level in response to what is purely a civilian law enforcement matter."
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"But does the act present a major barrier at the National Command Authority level to use of military forces in the battle against terrorism? The numerous exceptions and policy shifts carried out over the past 20 years strongly indicate that it does not."
Source:
www.homelandsecurity.org