In this essay, we trace the relations among the early years of the U.S.–Mexico borderlands after the Treaty of Guadalupe Hidalgo (1848), the role of racialist discourse in shaping the border and U.S. immigration policy, and contemporary bordering and security environments. Our ultimate aim is to show how contemporary security knowledge and practices form an assemblage with racialist discourses and practices in the post-9/11 era. Current security thinking is in itself racialized and follows the contours of what Étienne Balibar has described as “neo-racism” (1991), which has offered vigilante groups more credibility in matters of security and immigration than they previously enjoyed. In short, we will show how the racial–territorial nexus of “classical” racism has come together with the security–economy nexus of securitization theory and practice to form a neo-racist assemblage that we identify at the heart of U.S.–Mexico border security and migration debates.