Tell them you’re leaving and why. If you’ve already decided where to go, tell the other active players where you’re going in case they want to go too. Then go and don’t think twice.
Here’s a clearinghouse thread for teaching alliances. Hopefully in here you can find one that suits your playstyle and experience level
https://forum.smallgiantgames.com/t/teaching-alliances/39771?u=npnky