The main elements of U.S. immigration policy date back to the early Cold War. One such element is a screening process initially designed to prevent infiltration by Communist agents posing as migrants from East-Central Europe. The development of these measures was driven by geopolitical concerns, resulting in vetting criteria that favored the admission of hardline nationalists and anti-Communists. The argument proceeds in two steps. First, the article demonstrates that geopolitics influenced immigration policy, resulting in the admission of extremist individuals. Second, it documents how geopolitical concerns and the openness of U.S. institutions provided exiles with the opportunity to mobilize politically. Although there is little evidence that the vetting system succeeded in preventing the entry of Communist subversives into the United States, it did help to create a highly mobilized anti-Communist ethnic lobby that supported extremist policies vis-à-vis the Soviet Union during the early Cold War.