Why do modern states recruit legionnaires—foreigners who are neither citizens nor subjects of the country whose military they serve? Rather than exclusively enlist citizens for soldiers, for the past two centuries states have mobilized legionnaires to help wage offensives, project power abroad, and suppress dissent. A supply-and-demand argument explains why states recruit these troops, framing the choice to mobilize legionnaires as a function of political factors that constrain the government's leeway to recruit domestically and its perceptions about the territorial threats it faces externally. A multimethod approach evaluates these claims, first by examining an original dataset of legionnaire recruitment from 1815 to 2020, then by employing congruence tests across World War II participants, and finally by conducting a detailed review of a hard test case for the argument—Nazi Germany. The prevalence of states’ recruitment of legionnaires calls for a reevaluation of existing narratives about the development of modern militaries and provides new insights into how states balance among the competing imperatives of identity, norms, and security. Legionnaire recruitment also underscores the need to recalibrate existing methods of calculating net assessments and preparing for strategic surprise. Far from being bound to a state's citizenry or borders, the theory and evidence show how governments use legionnaires to buttress their military power and to engineer rapid changes in the quality and quantity of the soldiers that they field.