Total War: ROME II - Emperor Edition

Total War: ROME II - Emperor Edition

Alexander The Great Campaign Hellenic Edition
Melee Diver 11 Jan, 2018 @ 7:54am
The joining of Epirus
I have a question about one of the first bonus missions: "The joining of Epirus" it wants me to to make an client state of my Military Ally Epirus, wich confuses me a bit.
How am i suppose to make an Client state of them, am i suppose to lay waste to them by the sword?

Because this confused me so much i went to look up what happend in this period.
Phillip II had an Wife from Olympias of Epirus, whom is the Mother of Alexander the Great.
They also had an Daughter named: Cleopatra of Macedon who was married off to Alexander I of Macedon ( her Uncle ) and thereby cementing an alliance with Macedon.

Wich is happens to be the wedding at wich Phillip II got assasinated.

So if i got this right, Epirus should already become my Client state because Alexander's the Great, sister is ruling Epirus as Queen Regent.......

My dilemma is that i dont see the point in harming my military ally, only to subdue them into an client state.

And since i dont have the option to hand out my Sistser in marriage.....