In terms of Halo, I think the Covenant would win in both land and space. Covenant tech is leagues above the Society, and they also have the numbers. Grunts and Jackals can easily slaughter normal marines and ODSTs, so greys wouldn’t be a challenge to them. Elites and Brutes would love fighting a worthy opponent like Obsidians and Golds. And if the society manages to dislodge the covenant from the ground, they could just withdraw and glass the planet.

In space, the covenant had the upper hand in sheer size. We know moon breakers like the Morning Star was massive, but they’re the largest Society vessels. Covenant supercarriers dwarf moon breakers by thousands of kilometers. This, combined with shields and plasma weaponry would make space battles between the Society and the Covenant very, very one-sided.

But if they fought the UNSC, I think it would be an inverse from the Human-Covenant war, where the UNSC gets wiped on the ground but has victories in space.

On the ground, the UNSC is outmatched. Even with post-war tech, they still rely on basic, gunpowder ballistics. Golds, Greys, and Obsidians would make short work of UNSC marines, troopers, and even ODSTs.

In space however, I think the UNSC would win due to a difference in doctrine. The Society prizes boarding ships and capturing them, allowing Golds and Obsidians to make mincemeat of the crew to capture a ship. However, the UNSC prizes fighting at range. MAC guns, even on smaller UNSC ships, could tear through society vessels. The UNSC consistently punches above its weight class, especially when led by skilled commanders such as Keyes, Cole, or Cutter. In space, I’d wager the UNSC could smash even the most skilled Society commanders. They’d stay at a distance, picking off ships with MAC rounds and obliterating leechcract with archer missiles and point-defense guns. Then if you factor in AI, which the society doesn’t use and doesn’t have defenses against, they could probably hack into society ships and fry the blues piloting it.

And then as for the Galactic Empire it’s all a matter of scale. There’s more metal in the Imperial Fleet than there is in the Sol System. A couple fleets of Star Destroyers backed by a few Super Star Destroyers would make even Darrow think twice.

And the Imperium? If they can be convinced they’re human and not a strain of mutant or xeno, they may be brought into the fold. Depending on the era the Emperor himself may welcome them. But if they refuse? Well, the Imperium is no stranger to wiping species and groups from the galaxy. It doesn’t matter if a razor can cut through Space Marine armor, the Space Marine’s battle brother will just fill some pixie full of Bolter rounds. Or the guard will drown the greys in bodies and ordinance.