The Covenant has mobs of religious zealots, and the Empire has mass-produced infantry with armor-penetrating plasma weapons. The Covenant rely on energy shields, and the Empire has weapons that will tear through those defenses. The Covenant has big freaking ships...the Empire has big freaking ships and a mass of disposable, somewhat irritating fighters. And Sith. I think it'd be a good fight, but I think the Dark Trooper program is a match for the Elites, and the Stormtroopers are more than a match for the Grunts. Plus the Empire has superior assault mechs, I think. The Covenant has slow, vulnerable mechanical units (granted, the Banshee and Ghost are manuverable, but that's why you have a pack of TIE fighters).

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"4th Law of Modern Thermodynamics: Where Mihoshi is, Chaos Reigns." ~W. Hakubi
"Gun control is hitting your target; Recycling is reloading your brass." ~ Lex of Dirty Work.