"The Secretary of the Department of Homeland Security (DHS) is charged with preventing the entry of terrorists, securing the borders, and carrying out immigration enforcement functions. U.S. Customs and Border Protection (CBP), a component of DHS, has primary responsibility for securing the borders of the United States, preventing terrorists and their weapons from entering the United States, and enforcing hundreds of U.S. trade and immigration laws. Within CBP, the U.S. Border Patrol's mission is to detect and prevent the illegal entry of aliens across the nearly 7,000 miles of Mexican and Canadian international borders and 2,000 miles of coastal borders surrounding Florida and Puerto Rico. Although the military does not have primary responsibility to secure the borders, the Armed Forces generally provide support to law enforcement and immigration authorities along the southern border. Reported escalations in criminal activity and illegal immigration, however, have prompted some lawmakers to reevaluate the extent and type of military support that occurs in the border region. On May 25, 2010, President Obama announced that up to 1,200 National Guard troops would be sent to the border to support the Border Patrol. Addressing domestic laws and activities with the military, however, might run afoul of the Posse Comitatus Act (PCA), which prohibits use of the Armed Forces to perform the tasks of civilian law enforcement unless explicitly authorized."
CRS Report for Congress, R41286