The broad outlines of contemporary American immigration policy date back to the early Cold War. This article focuses on the screening process designed to prevent infiltration by communist agents posing as migrants from East-Central Europe. I argue that the development of these measures was a driven by geopolitical concerns and show how the vetting criteria favored the admission of hardline nationalists and anticommunists. The argument proceeds in two steps. First, I show geopolitics influenced immigration policy, resulting in the admission of extremist individuals. Second, I document how geopolitical concerns and the openness of American institutions provided exiles with the opportunity to mobilize politically. While there is little evidence that the vetting system succeeded in preventing the entry of communist subversives into the US, it did help to create a highly mobilized anticommunist ethnic lobby that supported extremist policies vis-à-vis the Soviet Union during the Cold War.