This book examines the U.S. law regarding use of the U.S. Armed Forces in domestic affairs. A brief scenario involving a terrorist attack using a weapon of mass destruction introduces an example of the dangers inherent in the use of military forces to regain or maintain public law and order. The discussion of U.S. law begins with the history of Posse Comitatus and proceeds through the passage of the Homeland Security Act of 2002. The Department of Homeland Security was created by this law and a subsequent action of establishing a unified commander with geographic responsibilities for the North American continent have been questioned as a breach of this law and a threat to civil liberties of American citizens. Upon examination of written law of the land and U.S. tradition, missions and functions of USNORTHCOM are consistent with U.S law and the constitutional duty of the federal government. Despite compliance with the law, involvement of military forces within our borders is contrary to U.S. tradition raising public concerns with the application and involvement of military forces. Concerns for USNORTHCOM are addressed at the operational and national-strategic level regarding the interaction of military forces with the civilian populations and public relations. Public perception driven by operations conducted within our borders will be a serious and continuing public relations challenge unique to the Commander of USNORTHCOM.