Why anyone would trust another realm to 'work together' is beyond me. "Oh, ya'll want me to help you attack that other realm...mmk..mmk, your entire army and theres is gonna be there, sounds nice...I'll certainly be there and wouldn't dare think of being opportunistic. Oh, double-teaming isn't opportunistic, that's different, I'm totataly your guy and think your realm should win the island!"
Roll right into your 'war-friend's capital because they showed their hands and you didn't.