Posse Comitatus and Martial Law
The Posse Comitatus Act of 1878 prohibits the use of U.S. military forces to perform the tasks of civilian law enforcement such as arrest, apprehension, interrogation and detention unless explicitly authorized by Congress.
The Posse Comitatus Act was originally enacted due to the feeling of many members of Congress at the time that President Abraham Lincoln had exceeded his authority during the Civil War by suspending habeas corpus and creating military courts with jurisdiction over civilians.
It should be noted that the Posse Comitatus Act greatly limits, but does not eliminate the power of the President of the United States to declare "martial law," the assumption of all civilian police powers by the military.
The president, under his or her constitutional powers to put down insurrection, rebellion, or invasion, may declare martial law when local law enforcement and court systems have ceased to function. For example, after the bombing of Pearl Harbor on December 7, 1941, President Roosevelt declared martial law in Hawaii at the request of the territorial governor.
What the National Guard will not do on the Border
The Posse Comitatus Act and subsequent legislation specifically prohibit the use of the Army, Air Force, Navy and Marines to enforce the domestic laws of the United States except when expressly authorized by the Constitution or Congress. Since it enforces maritime safety, environmental and trade laws, the Coast Guard is exempt from the Posse Comitatus Act.
While Posse Comitatus does not specifically apply to the actions of the National Guard, National Guard regulations stipulate that its troops, unless authorized by Congress, are not to take part in typical law enforcement actions including arrests, searches of suspects or the public, or evidence handling.
What the National Guard will do on the Border
Operating within the limitations of the Posse Comitatus Act, and as acknowledged by the Obama administration, National Guard troops deployed to the Mexican Border States will, as directed by the states' governors, support the Border Patrol and state and local law enforcement agencies by providing surveillance, intelligence gathering and reconnaissance support. In addition, the troops will assist with "counternarcotics enforcement" duties until additional Border Patrol agents are trained and in place. The Guard troops may also assist in the construction of roads, fences, surveillance towers and vehicle barriers necessary to prevent illegal border crossings.
Under the Defense Authorization Act for FY2007 (H.R. 5122), the Secretary of Defense, upon a request from the Secretary of Homeland Security, can also assist in preventing terrorists, drug traffickers, and illegal aliens from entering the United States.
Posse Comitatus: Where Congress Stands
On Oct. 25, 2005, the House of Representatives and Senate enacted a joint resolution (H. CON. RES. 274) clarifying Congress' stance on the effect of the Posse Comitatus Act on the use of the military on U.S. soil. In part, the resolution states "by its express terms, the Posse Comitatus Act is not a complete barrier to the use of the Armed Forces for a range of domestic purposes, including law enforcement functions, when the use of the Armed Forces is authorized by Act of Congress or the President determines that the use of the Armed Forces is required to fulfill the President's obligations under the Constitution to respond promptly in time of war, insurrection, or other serious emergency."