Right, but after 100 turns of alliance you don't just stab him in the back do you? There's gotta be a way for either a joint win or a singe (2 outa 3, whatever) contest that will choose the victor.
I'd say either do an arranged apocalyptic battle with everything, 1v1 agreed upon SCs, or 1 naked h1 priest vs another
If the alliance got you there, its kind of crappy to have the guy with treachery in mind the last 10 turns positioning units and hoarding resources suddenly go boo.