Today’s US cybersecurity milieu can be traced, in part, to the encounter between electronic technology and border control during the 1970s. Federal officials argued at the time that, due to the end of the Bracero Program and passage of the Immigration and Nationality Act of 1965, “illegal aliens” constituted a “silent” and “invisible invasion” threatening the US nation. In the science of cybernetics, officials found the conceptual tools to organize border enforcement operations and institute order over border life. Intrusion detection systems installed on the southern border area in 1970 highlight how actors attempted to control racialized populations by managing information and communication processes. Cybernetics and intrusion detection systems, this article argues, aimed to draw an electronic “line in the sand.” They were put to work in making the nation’s boundaries both on the ground and on human bodies; in doing so, they reproduced US racial sorting logics while recoding them into bits of data. The “electronic fence,” as the system was known, was part and parcel of an infrastructure of technologies and knowledges targeting Mexicans. Tracing discourse networks in government publications and memoranda, technical reports, and newspapers, the essay charts the role of automation in US nation-making and in the management of racialized bodies.