Maybe the treaty isn't benefiting everyone, or at least some people don't think it is. Maybe the treaty is restricting trade to between those cities only and the antagonist wants to get in on some trading biz with some other country. He tries to do so illegally, but he gets caught and put in jail for a long period of time and his business goes down the drain and his wife and kids starve to death.
So when he gets out of prison he wants to make a few changes. Taking over other cities would allow him to control the markets a little easier and control the local police and allow him to get into that trade biz with the outside world.
This is the approach I would use. The existing alliance set up is very profitable to a substantial minority in each of these fiefs, but wrecks havoc with the majority - farmers forced to become serfs, laws written so that merchants who don't have an 'in' with the Alliance don't do very well at all, so on and so forth.