In Warcraft 2, you used to be able to change your allegiances mid-game. This doesn’t seem to work (well) anymore in the remastered version. One thing I definitely observed is that disabling ‘allied victory’ does nothing. When the other team is defeated, you still win. But, my ally then does appear as ‘enemy’ in the score screen.
We tend to turn on each other in cooperative play against computer players, because that gives an interesting dynamic. You never know if your allies will turn on you, and this make planning a strategy far more interesting.