Since the mid-19th century, the United States has frequently employed the US 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 antidrug efforts. 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 paper reviews the lengthy history of the US Army on the Mexican border and highlights recurring themes that are relevant to today's ongoing border security mission. Between 1846 and 1921, the US Army carried out its security missions under a variety of hardships imposed by the massive length and ruggedness of the border. The shortage of soldiers to police the new and oft-disputed border also proved especially problematic. Mexican domestic politics and US-Mexican international relations greatly affected the Army's operations. Since 1921, the Army's role has been dramatically different, ranging from noninvolvement to varied forms of support to local, state, and Federal civilian agencies. The narrative brings to light these complexities and makes for compelling reading. The ongoing, post-9/11 debate over the military's role in securing our Nation's southern border makes this paper important reading for today's soldiers. While current and future missions will not mirror those of the past, the historical record is replete with insights and lessons learned from the Army's past that are timely and relevant today.