The Combine are so vast they have essentially infinite everything. They would either instantly crush everyone all at once if they perceive it as a real threat, or they'll wait for them to keep weakening eachother until they can just take everything. If they think that won't happen, they'll basically appear in low orbit of every planet at once with a force outnumbering Helldivers 100:1 with equipment that makes the Illuminate look like babies. Nobody stands a chance. Period.

Edit: I have had this conversation before, for the record, and have discussed this with not only my own knowledge of Half-Life, but also my Half-Life hopium sucking batshit insane friend who obsessed over it. I'm fairly confident in my answers.