The attacks of 11 September 2001 showed that America has problems with its immigration system. The nineteen hijackers involved in these attacks took advantage of a problematic immigration system to penetrate the U.S. and wreak havoc. The porous border between the U.S. and its neighbors is another means by which terrorists can enter America. This thesis examined the legal permissibility of using federal troops to protect the nation's borders. It explored the growing problem American has with illegal immigration and how that problem is a threat to national security. The only viable solution to the border security problem is an increased presence along the border. The military is a source of manpower to supplement the United States Border Patrol (USBP). Asking federal troops to assist with border security, though, can create problems with the Posse Comitatus Act (PCA). This thesis used a legal methodology to examine this issue. The legal material analyzed in the course of this study showed that, barring a change in the PCA, the most that federal troops can provide is limited.