Ask them to end their war Revoke their titles Or kill relevant claimants in claimant wars
Alternatively, if you want to prevent vassals from fighting in the future, you can increase your Crown Authority or keep vassals as small and weak as possible.
How To End Vassal Wars
#1: Ask Them To End The War
In some cases, you can simply ask a vassal to end a war they have declared. Ask them to end the war or surrender, and they may stop. Some vassals may refuse if they want to continue the war or don’t like you. You can use hooks here to increase your chance of success.
#2: Revoke Their Titles
If you revoke a vassal’s title, you might be able to end their war. You would have to revoke all of their titles; titles that are the target of a war cannot be revoked. Note: you will need Crown Authority Level 2, or Level 3 for Tribal governments, to revoke titles from vassals. Their vassal contract cannot have “protected from title revocation” checked. This action gives you Tyranny, which will cause all vassals to dislike you, so be careful. If a vassal has committed a crime that allows you to revoke a title as punishment, you won’t gain any Tyranny. Sometimes, vassals will rebel against you when they don’t want you to take their titles! They will declare war with any other vassals who dislike you. Again, you can use a hook here to help convince them to give up their titles.
#3: Kill Claimants & Their Family In Claimant Wars
The goal of a claimant war is to replace a ruler with another character, who the supporters would prefer to rule. In a claimant war, you can kill the claimant and their family, and the war will no longer have a purpose. This ends the war inconclusively. This may be difficult if your vassal is from an established family. If their title is inherited when they die, the war will continue with the next ruler.
How To Prevent Vassal Wars
#1: Increasing Crown Authority Level
Crown Authority Level 3 will prevent vassals from declaring war on each other. This will put a stop to internal wars, which will restrict vassals from gathering land and power. Crown Authority Level 4 will prevent vassals from declaring any kind of war, which includes wars against internal vassals and external realms. If your vassal has a strong hook on you, they can still declare any type of war they want. Note: Tribal governments cannot regulate war declarations. Tribal vassals are free to declare both internal and external wars. Avoid giving vassals the Sanctioned War Declaration option in their vassal contract. This will allow them to declare war despite Authority levels, just like hooks! Faction-based wars can still happen no matter what restrictions you’ve put in place. This is because factions are rebelling against those restrictions and you, as their ruler.
#2: Keeping Vassals Small & Weak
By the time you can control your warring vassals, it’s likely they’ll have expanded and become stronger already. Controlling vassals and keeping them weak will stop them from forming factions and attacking you as often. It will also discourage them from waging war against other vassals or realms. As mentioned earlier, you can revoke titles from vassals, ideally as a punishment for their crimes. You can also modify vassal contracts to control their power. Add the Forced Partition option to make their lands split between their children when they die. Or, increase their Feudal Levy obligations to lower the strength of their armies.
