Okay, let's take the precursors AND the gods of Star Wars out of the equation to make the fight winnable for either side... The Covenant can build ships that act like the Death Star, and those ships travel at the speed of light, plus their weaponry is on par, if not more advanced, than the Empire. The UNSC would destroy the Rebels sheerly through mechanical possibilities alone. Then there's the Flood, which would destroy that one race of organic machinery... And the forerunners could wipe of whatever's left with the Halo Rings. Now, I know your argument about the Force, and Jedi and Sith, and yes, they would put up a serious fight, and would make it hard for the UNSC or the Covenant to kill them, but, the Flood would overwhelm them, or the Forerunners could nuke them with the Halo Rings. But, that's my opinion and break down of it. Have a nice day, everyone.