How to determine if the ally will join the side of a war

europa-universalis-iv

In the case of an alliance à trois, is there a way to determine which side the mutual ally will join?

For instance, as The Palatinate, I am currently allied with Wurttemberg, but so is Alcase. If I declare war on Alcase, is there a way to know which side the mutual ally will throw in with? If so, is there a way to influence that decision?

I thought it may be based on which side they liked more, but I had my rep at 200 with them and they decided not to join. Is it based on the reason for the war, relative power, or something else entirely?

Best Answer

Generally speaking, the common ally will always side with the defender, regardless of relations; you can rarely (if ever) expect them to join on the offensive against their other allies. That's only valid for wars you declare on Alsace directly; countries might find themselves in strange combinations in wars brought by their other allies. Most likely, you won't be able to exploit such situations unless you or Alsace happen to be become a war leader in a third-party conflict (assuming The Palatinate in your game is a minor or at best regional power).

What you can do instead is ally with Alsace's rivals -- they will almost always join a war against them unless they are fighting alongside them in another war or have a truce in effect. This will of course mean fighting against Wurttemberg as well, but hopefully your other ally can throw some punches to offset that.