It's easy to imagine the circumstances: A social function, gathering that worldly stratum of government officials whose members go by titles such as consul, attaché, and ambassador. The drinks are flowing, the pressures of one's post are at their most acute, and yet the wisest of the group know well that conducting sensitive state business could be gravely jeopardized by dulled faculties.
Owing to its spirits-less build, this cocktail is relatively low in alcohol, and so the diplomats who favor sipping Diplomats can keep ahold of their wits even while appearing to indulge graciously in their host's hospitality.
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."
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, and I think that's a good idea, for the sake of balance. For those who want to delve into the complex flavors of fine vermouth, the Diplomat can arrange such passage. Expertise in international affairs not required.