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You can break an alliance quickly by declaring war against your ally.
Alternatively, divorcing or breaking betrothals that formed the alliance will also end it. Or as a last resort, you could murder characters from these marriages to end the alliance too.
Method 1: Declaring War Against Your Ally
This is often the quickest method of breaking an alliance.
Whenever you declare war against an ally, your alliance with them will be broken, and you’ll lose Prestige and Fame.
This method can be inconvenient since you must actually fight in a war. You can optionally start the war and immediately surrender to end it, although this costs Gold and Prestige.
You can also opt for a minor war if it’s available to you, such as a Raid for Captives if your religion has Human Sacrifice.
Method 2: Divorcing Ally Marriages
Most alliances in the game are formed by marriage ties.
Usually, an heir or sibling is married to a family member from another realm.
If you still have the option of forcing a divorce or breaking a betrothal, this is the easiest method.
Not all players will have the option to divorce, however. You need a religion that allows divorces to happen.
If your own marriage to your spouse has created an alliance, it’s easier to divorce personally compared to forcing your heirs to divorce.
Method 3: Murdering Ally Spouses
Of course, if the other methods don’t work for you, you can always kill one of the spouses involved in the alliance.
Ideally, this should be the ally’s family member. But you can also murder your own heirs to achieve the same result.
Be careful if you’re murdering your family, since this often comes with heavier penalties if you’re caught.