I think this question is inherently flawed due to the war taking place between the entire Covenant vs the singular Red Legion. Of course the Covenant have a massive number advantage, because you brought a whole empire against a single battalion within an empire, but if this was a war between the Covenant and the entire Cabal Empire, not just the Red Legion, then we have working debate. Due to a lot of the comments below, I'm led to believe that the Cabal would win against the Covenant, and I don't say that because I'm a Destiny fan. Knight_of_Alpha nailed it pretty well, I dont need to repeat what he said, but everyone shouldn't underestimate the Cabal's ability to analyze, grow, and adapt to conquer enemies. If the Covenant went after the Traveler, I bet my PS4 they would've ran straight in with their forces to claim their prize, after striking key positions like the tower. The Covenant arent really known for subtlety, not when it comes to all out war. But the Cabal didn't do that, not only did they collect surrounding technologies to improve their own forces, but they created a perfect attack plan to conquer the City and the Traveler. The Guardians didn't save shit, the only reason we still exist is because the Traveler woke up to save us one last time. Even aftet the whole story line, in the end the Guardians were powerless to stop Ghual, and the Traveler knew it. The Traveler didn't wake up for the Vex, didn't wake up for Oryx, didn't wake up for any previous disasters, but it woke up for the Cabal. I honestly think that at the start of the war, the Cabal would be somewhat taken by surprise by the new enemies and their tech, but could hold their own due to advanced training and tactics, not to mention that the common Legionnaire can easily best the common Grunt. The Covenant would lose in a prolonged conflict against the Cabal primarily due to their lack of change vs the Cabal's constant evolution within the course of a war.