Since the mid-19th century, the United States has frequently employed the U.S. Army on its southern border to perform various roles in support of the Nation, from outright war, to patrolling the border, to chasing bandits while securing persons and property on both sides of the border, and most recently to supporting civil law enforcement and anti-drug efforts. The military generally provides 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 re-evaluate the extent and type of military support that occurs in the border region. Events since 9/11, such as the recent deployment of National Guard Soldiers to the Mexican border, are only the latest manifestation of this long tradition. This book reviews the lengthy history of the U.S. Army on the Mexican border and highlights recurring themes that are relevant to today's ongoing border security mission. This book consists of public documents which have been located, gathered, combined, reformatted, and enhanced with a subject index, selectively edited and bound to provide easy access.