When at war against another character, the subjugation action becomes possible if all the territories of that character are occupied.
It is only available to players at war with the character, and in control (occupation) of at least 1 of his territories.
This action automatically ends the war between the 2 foes and integrates the defeated player into the nation of the victor.
It also automatically makes peace between the defeated player and all the other members of the victor's nation.
Breaking clauses are asymmetric: the vassal has a GDP penalty while the victor gets an honor commitment to withhold the peace.