We already know that the Cabal on Mars are not part of the main group, and may even be a rebel force or an offshoot of the main army. I would have to guess that a war between the main army and the Mars occupants might be coming in a future dlc. Perhaps the homeworld Cabal will be a little more reasonable when dealing with us, (especially given the events that are sure to unfold in TTK)
-
Cabal are actually allianced with the Fallen, we just don't see it since the Fallen aren't on Mars