Birthright was a setting in Advanced Dungeons and Dragons (so second edition ish?) with rules for playing like countries more than adventurers. Well heads of state and the such with politicking. I never played, but my husband did.

It wasn't the most well known setting and I don't think it ever got any updates unlike other settings (like faerun or ebberon etc) but it fits your idea of playing as nation states with tweaking!