Abstract Conventional accounts of a drastic shift to migration restriction after World War I following a golden era of free movement obscure crucial processes of state formation around matters of administering migration. How and with what consequences did state control over migration become acceptable and possible after the Great War? Existing studies have centered on core countries of immigration and thus underestimate the degree to which legitimate state capacities have developed in a political field spanning sending and receiving countries with similar designs on the same international migrants. Relying on archival research, and an examination of the migratory field constituted by two quintessential emigration countries (Italy and Spain), and a traditional immigration country (Argentina) since the mid-nineteenth century, this article argues that widespread acceptance of migration control as an administrative domain rightfully under states' purview, and the development of attendant capacities have derived from legal, organizational, and administrative mechanisms crafted by state actors in response to the challenges posed by mass migration. Concretely, these countries codified migration and nationality laws, built, took over, and revamped migration-related organizations, and administratively encaged mobile people through official paperwork. The nature of efforts to evade official checks on mobility implicitly signaled the acceptance of migration control as a bona fide administrative domain. In more routine migration management, states legitimate capacity has had unforeseen intermediate- and long-term consequences such as the subjection of migrants (and, because of ius sanguinis nationality laws, sometimes their descendants) to other states' administrative influence and the generation of conditions for dual citizenship. Study findings challenge scholarship that implicitly views states as constant factors conditioning migration flows, rather than as developing institutions with historically variable regulatory abilities and legitimacy. It extends current work by specifying mechanism used by state actors to establish migration as an accepted administrative domain.