I think this is a problem unique to starter alliances (0 cups no invite types).
I think a lot of new players make mistakes when they play their first war - they don’t realize what they are getting into, and then don’t realize they are automatically in future wars until they opt out. I think some sort of follow up prompt would help after the first war.
“Would you like to participate in all future wars (you can choose to participate at any time if you choose no now)?”
The problem for starter alliances - if you have a “use all flags requirement”, and they can’t, not a big deal, they figure it out, that’s how you learn, but then next time they are in war and may not even realize it, and then it becomes a problem for the alliance - do you kick the player to stop them from bringing the active players down or keep giving them chances?
At least if there was a follow up prompt they would realize they needed to opt out or that they were opted in, I guess. This is similar to other war requests, but I think an alternate way of handling it.