Didn't you hear? The Optional Defense Network is rubbing off on them.
I kid, of course (Because I do <3 ODN) but I understand that conflicting treaties DO get frustrating. My only suggestion here is this: When you (and this goes for everyone in general) go to sign a treaty with another alliance, make sure their other treaty partners are people you can see yourself getting along with and having similar views to when it comes to wars. That way, situations like this one can be partly mitigated.
Hope that helps. :/