Foreign soldiers were a major element in virtually all European armies between the early sixteenth and mid-nineteenth centuries. The extent and duration of their use clearly indicates they were far more than a temporary expedient adopted solely until states acquired the capacity to organize forces from their own inhabitants. Rather than being a hindrance to state formation, they were integral to that process. Likewise, the formation of European states and an international system based on indivisible sovereignty was not purely competitive: it also entailed cooperation. The transfer of foreign military labour is an important example of this and is central to what can be labelled the European Fiscal-Military System, which assisted the emergence of a sovereign state order and was dismantled as that order consolidated in the later nineteenth century. Wilson’s article articulates ‘foreign soldiers’ as an alternative to the problematic term ‘mercenaries’, and examines their motives, explaining how and why foreign soldiers were recruited by early modern European states.as well as assessing the scale of their employment. The article concludes that the de-legitimation of foreign military labour was connected to fashioning the modern ideals of the citizen-in-arms as part of a more general process of nationalizing war-making.