The new Vassal States feature allows players to take up other empires as "vassals," the game's equivalent of protectorates. When an empire becomes a vassal, it loses the ability to declare war and make peace independently, and may be called upon to pay "tribute" in the form of game resources to its "master" (suzerain) state in return for the promise that its master will protect the vassal. Players can use Vassal States to achieve a Domination Victory, since half the vassal's land and population count towards a domination victory for the master, but not vice versa. Similarly, if the master goes to war with another civilization, the vassal too has to go to war. However, other countries' opinions of you will worsen if you have a vassal they dislike. The vassal may still reject trades for gold and technologies.
During peacetime, civilizations can offer their vassalage in return for protection from an enemy, but it may be ended if the state's relative land and population statistics change. Vassal agreements signed in a state of mutual war, as part of a peace treaty, are considered capitulation and bind both parties. The agreement is terminated, however, if the vassal acquires 50% more land area than the master or the vassal loses half of the land it held when the agreement was signed. The only way a master can terminate the agreement is if the vassal refuses to pay tribute. The master can then choose to declare war.