IIRC, the actual guys who conquered Earth left a while back, and you are fighting the losers stuck in garrison duty over the course of HL2.

Because that's not the Combine forces that conquered Earth. The Combine soldiers on Earth are just modified humans garrisoning the planet that receives some meager support from the Combine in the form of striders, ships, hunters, etc etc, when they feel like it.

Anon would easily realize this from inferring through the fact that the Combine obviously didn't invade Earth with human soldiers, but media literacy is dead on 4chan.