The construction of a US-Mexico border fence has proved an increasingly divisive method for securing the southern US border since its initial construction as the San Diego Fence in 1990. Since the terrorist attacks of September 11, 2001 the construction of a US-Mexico border fence has morphed from a method to stem the tide of illegal immigration from Mexico into a legislated method to prevent the entrance of terrorists. All terrorists except two have gained entrance into the US through manipulation of the US immigration system. Those two exceptions crossed the US-Canada border. This thesis analyzes what threat Mexico poses as a transit for terrorists attempting to enter the US by surreptitious means, and what deterrent or defeat effect a US-Mexico fence provides. The absence of a terrorist attempt to gain entrance into the US via Mexico indicates that Mexico does not provide the essential support required for an Al Qaeda terrorist, and that other countries and other means of entrance provide a more viable option and likelihood for success. The one support element that could provide a terrorist entrance from Mexico is that of the "coyote". A strategy of vigorous enforcement against illegal immigration without combating the motives for illegal immigration has helped to enable the "coyote" to gain the capability to transport a terrorist into the US from Mexico. The US-Mexico border fence has a limited deterrent effect against terrorists as demonstrated by the migration of illegal immigrants to more remote crossing sites. Finally, this thesis provides recommendations to better secure US land borders.