"The concept of Posse Comitatus is well understood by all three branches of the government. As a statute, the Posse Comitatus Act of 1878 prohibits the use of U.S. military forces to enforce civil law without a Presidential Proclamation of National Emergency. Drug interdiction requirements in the U.S. have exceeded the capability of civil authorities at all levels, federal, state, and local. The Congress mandated that the Department of Defense take the lead in this area. This places the military in the difficult position of executing a mission that encroaches on the intent of Posse Comitatus. The National Guard, by virtue of its unique dual role as a federal force, and as a state militia, is exempt from some of the restrictions imposed upon the active military. As such, the Guard is being committed to training missions under Title 32 of the U.S. Code in direct support to civil authorities. The extended use of the Guard in a drug interdiction role appears to violate the intent of Posse Comitatus. Such an interpretation by the courts could obviate any success achieved by DOD [Department of Defense] in attempting to accomplish its mission."
Defense Technical Information Center (DTIC): http://www.dtic.mil/dtic/