The Diplomat dates back to at least 1922, the year it was published in bartender Robert Vermeire’s Cocktails: How to Mix Them. In that volume it’s named “Diplomate”; Vermeire seems to explain away the unusual spelling by noting that the drink “is very well known in the French Diplomatic Service.”
Owing to its spirits-less build, this cocktail is relatively low in alcohol, and so the diplomats who favour sipping Diplomats can keep hold of their wits even while appearing to indulge graciously in their host’s hospitality.
Numerous tweaks on the recipe exist, but the basic formula features both dry and sweet vermouth and a small measure of maraschino liqueur. Most later recipes call for bitters too.