The threat of "blowback" from foreign fighters, unaffiliated volunteers who join an insurgency in a distant land, has led states to explore a variety of normative mechanisms. Among these is the international legal regime applicable to mercenaries. This short article considers the evolution of mercenarism and the efforts to regulate it. Attempts to fit foreign fighters into that normative category are unlikely to succeed. In part this is due to the question of motivation, which is central to most definitions of mercenary and focuses on private gain. But it is also linked to the reasons for regulation in the first place: mercenaries are seen as threats in the states to which they travel, while foreign fighters are primarily deemed threats by the states to which they might return.