@the dude man, you mentioned earlier of our supreme ballistic capabilities. Well imagine that the combine, as an advanced race and what not, did have some kind of energy shields and weapons, more powerful of those used by the Strider and the defense walls in City 17. As everyone already said, what we see here on Earth is an occupation force, constructed mostly from our own people and resourses, spiced with some alien tech.
If you were to conquer some small planet, full of apes with slingshots (our great ballistic guns in their eyes), would you leave your elite forces to protect this? Of course not! And about those rpgs and machine guns, pistols and whatever, in Half Life 2. If there is so much alien construction material laying around, who said the bullets are not made from it, to harm it. Ever watched Falling Skies? And in that way of thinking, remember War of the Worlds? Yes, nowadays striders and such don’t have shields, but imagine back in the 7h war a Mothership, with some kind of massive energy reactor, charging units from above with energy shields.
And what if we see some new forces in ep3, that will make more clear the shit behind the 7h war. Anyhow, City 17 is not the only Combine base on Earth. Who knows what, Nova Prospect-like facilities lay around the world.
Also, they never knew about Gordon and his capabilities. If they knew, they would leave better occ force. He was like a thunder from a blue sky.
Uh, and imagine this! Our most superior weapon is the nuke. If we tried that on the combine forces (that we have no idea how they look like or of what were they consisted) it would be like throwing biscuits and milk at them.
Lol, nukes?? Great, they are charging our weapons! Thanks USA!