This work examines the use of the U.S. Armed Forces in civil authority support missions along the U.S. -Mexico border from the creation of the boundary between the U.S. and Mexico in 1848, to the post -September 11th border security support operations. Many questions arise from using the military in this capacity, for example; how effective is the military's support to civil authorities (MSCA), can the military perform MSCA operation without the threat to human and rights of civilians, how do MSCA missions such as these impact the combat readiness of the military? This work found that military support to civil authorities was indeed effective, especially with in cases where non -federalizes National Guard personnel were employed. It also shows that the military has implemented control measures that enable troops to conduct law enforcement support missions while respecting the human and civil rights of civilians. Lastly, it found that the combat readiness the military was not necessarily diminished, but could actually be enhanced during MSCA operations