The 19th-century disappearance of mercenary forces within Europe is generally regarded as a cornerstone in the historical sociology of the international as it provided important conditions of possibility for the historical consolidation of the state’s monopoly on violence. Contrary to the claim that mercenaries disappeared in this time frame, this article argues that the mercenary concept was constituted within it. The argument unfolds in three steps. First, it revisits a central dichotomy within this narrative, the soldier/mercenary dualism, exposing the conceptual ambiguity surrounding these terms before the 19th century. Second, drawing on the work of Michel Foucault, it turns to an overlooked dimension of the 19th-century transformation in military force, namely a change in the conditions of knowledge and the constitution of the modern subject grounded in the modern state. Last, by investigating 19th-century discourses on ‘effeminacy’ and the formation of the social, it exposes how the mercenary enters into reflective practice as an actor threatening to undo the compact between man and state, by being simultaneously a symptomatic expression of, as well as cause for, the erosion of the masculine principle upon which this compact rests.