You think all alliances have stuck through thin and thick in the real world? nope, before ww2, america had plans to invade and conquer britians army, then after, they were forced to became dick sucking allies cause if they didn't nazi germany would of handed their ass to them after they dealt with england. etc etc.
There are no rules against betraying, it's a gameplay feature, if you don't like it, expand better than your ally, or don't play the game.