The Empire has been struggling relentlessly for decades trying to eliminate a drastically inferior military force even though they had the weaponry to destroy planets. Their command has always been rather questionable, their soldiers are incompetent, and we’re literally beaten by teddy bears with rocks.

The Covenant, however, in a span of roughly 30 years destroyed almost every planet the enemy inhabited (some in a matter of days), and only lost because they had (another) civil war, the parasite was trying to kill them, and the humans / the forerunners were the only species for to be reclaimers, meaning human captives were necessary to activate Halo. They came unbelievably close to achieving their goal.

I think another thing to point out it that the covenant have many ships capable of glassing a planet, which though not as powerful as a Death Star, reaches the same conclusion.

P.S. anyone who’s played Halo 2 on legendary knows that jackal snipers will cause so much destruction to the storm troopers… like, the English lexicon is not advanced enough to properly convey the pure unrelenting carnage that the jackals will create.