En Garde! is a RPG set amongst the French military in the era of the Three Muskeeteers, which is one of the first games I remember to have a substantial element of social interaction as part of the requirements. As the first edition was published in 1975 it is definitely one of the first RPG to look at this, most other games at the time were much more from wargaming perspectives, particularly D&D and its imitators where morale is the main psychological factor considered.
I have rarely done what this game is really good for, which is extended campaigns having generated far more characters than I ever played (not unusual with a lot of games systems) and used the duelling rules quite often even for other games when it seemed more appropriate than those games core ruleset (neither Traveller or D&D gives a particularly good feel for that intense one on one combat like a duel).
The requirement for characters to keep a diary of events makes it particularly suitable for play by mail or play by post games and the continuing support has been from the pbm community and now the online community. There have been variations in setting used from the fictional Paris of the original to London or Rome. There are active games in many languages and the official site lists them.