Foes that are a credable and interesting military threat

[Tasoth]

For the comment about the heavier combine forces curbstomping Earth, that would be the crab and mortar synths. You get to see them during one of the little parades through the citadel stomping acrossed various catwalks. The mortar synths, as the same suggests, hurled explosive shells, don't know how explosive, but probably good enough to threaten infantry. The crab synths were supposed to have brutal physical attacks. If they have enough strength to break the tracks on a tank, bend its gun or flip/penetrate lighter vehicles, they'd be frightening. Couple that with the synths apparently being hover based and able to appear literally on top of you thanks to the portals, even a defensive position can turn into a slaughterhouse.

[Darth Smiley]

I do get the feeling that the current forces on Earth are really just occupiers, not the actual world-conquering force.

Two things we need to get straight here : The Combine [/b]DO NOT[/b] have the ability to create portals from one point to another. This was a major point in HL2, and it was something that the Combine were working on.

My theories as to why the didn't already have the technology are that
1) The did, but don't trust the Breen administration with them, as they are a potential threat to the Combine and therefore must be kept out of the slaves' hands at all cost. Breen was working on teleports as a method of gaining leverage with the Combine
2) The didn't because none of their previous conquest had developed it. The Combine probably doesn't do it's own R&D, it just takes the best from whoever it conquers. With the complete assimilation of humanity, they would have added local teleport capability to their bag of tricks.

The other things is that the Combine DID NOT use any kind of chemical agent to force sterility. Instead, it was some sort of energy field, which collapsed after the events of HL2 due to damage done by Gordon Freeman. There is an announcement during episode one where Kleiner advises everyone to "get busy", in case the field is reestablished.

The reason the Combine can't use this field as a "deathwand" is that their own technology is organic, and therefore any attempt to disable basic biological functions would kill everyone, Combine included. Of course, I wouldn't put it past them to create a 'doomsday device' with this technology, in case the planet ever successfully rebelled.....

[Gil Hamilton]

For the comment about the heavier combine forces curbstomping Earth, that would be the crab and mortar synths. You get to see them during one of the little parades through the citadel stomping acrossed various catwalks. The mortar synths, as the same suggests, hurled explosive shells, don't know how explosive, but probably good enough to threaten infantry. The crab synths were supposed to have brutal physical attacks. If they have enough strength to break the tracks on a tank, bend its gun or flip/penetrate lighter vehicles, they'd be frightening. Couple that with the synths apparently being hover based and able to appear literally on top of you thanks to the portals, even a defensive position can turn into a slaughterhouse.

I don't know about that. We never engage them in the games, as far as I know. Frankly, unless they are significantly more powerful and armored than Striders, I don't see them being a Conquered Earth in Seven Hours.

After all, it doesn't matter how strong a Crab Synth is if a tank shell will bust it before he can get its claws on the tank. And Striders and Gunships are already pretty good at dicing infantry as it is. You wouldn't make it out of that courtyard during the Three Strider Battle if it weren't for the fact that Gordon Freeman has the HEV suit and thus is a one man warmachine.