Posse Comitatus

passed on 1878-06-18 after the end of Reconstruction (US Civil War). The Act was intended to prohibit Federal troops from supervising elections in former Confederate states. It 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 US Constitution or US Congress. The Posse Comitatus Act and the Insurrection Act substantially limit the powers of the Federal government to use the military for law enforcement. http://en.wikipedia.org/wiki/Posse_Comitatus_Act

limits/exclusions http://en.wikipedia.org/wiki/Posse_Comitatus_Act#Limits_on_the_Act