The introduction lays out the book’s main arguments and themes, and compares the Royal Navy to the two different contexts which it straddled: the maritime world, and the armed forces. Naval service was more strictly regulated and anchored in the structures of the state than work in the merchant marine, and it was invested with explicit national and patriotic meaning. However, it also differed from service in the Army, as it required a good proportion of recruits to have specialised skills, and usually integrated them all into mixed crews, rather than establishing separate ‘foreign’ units. The Navy’s peculiar status, suspended between the military and national on the one hand, and the maritime and transnational on the other, is what makes it an important case study. If ‘foreign Jack Tars’ were in some senses mercenary fighters, they were also primarily – like ‘British’ Jack Tars themselves – a transnational, mobile, and often highly professionalised seafaring workforce. Studying them in the crucial historical juncture of the French Wars allows us to present a transnational history of a national institution, expose the compromises and contradictions underlying the power of modern states, and probe and deconstruct the very meaning of the term ‘foreigner’.