@Tricon: I think they couldn´t be seen as mercenaries. Were the Hessians part of the British Army? (the term "sold" suggests, that they are property of the English king ;) ) If so, then I´d say no because of letter (e) of the definition.
If they were in America as Hessian troops, then letter (f) of the defintion would apply. Alternatively Hessia could be considered a Party of the conflict (letter e), because they actively supported the Royalists.
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