Combine History
WARNING: SPOILERS WITHIN
Introduction
The Combine is an intergalactic (or possibly interdimensional, or both) alliance of species. Its true nature is not very well known. What we do know is this:
- The Combine were in control of Xen at the begining of Half-Life 1.
- The Vortigaunts had been enslaved by the Combine.
- During the course of of Half-Life 1, Gordon Freeman freed the Vortigaunts from Combine oppression, likely by killing Nihilanth, who seems to be equateable to Dr. Breen on Earth.
- The Combine invaded Earth after the Portal Storms, and took 7 hours to defeat all of Earth's armed forced. This was known as the 7 Hour War.
- After the invasion, Dr. Breen brokered a "peace" with the Combine and convinced them that Humanity could be evolved to the point where it would be useful enough to be included into the Combine. Dr. Breen worked with the Combine to produce "trans-humans", mostly in the form of Combine Soldiers.
- The Combine had at least one enormous skyscraper-like HQ at the beginning of Half-Life 2. It's possible there may be more, possibly one for each of the cities, such as City 13, mentioned at the beginning.
- The only other race apart from the natives of Xen and Transhumans is the race of the Combine Advisor.
Before HL1 - Xen
Before HL1 the Combine had already invaded, conquered and pacified Xen. It seems that the natives of Xen, the Vortigaunts, were enslaved, which is mentioned throughout HL2. Also, the entity used to place Vortigaunts in HL1 was called monster_alien_slave.
Why exactly the Combine invaded Xen is a mystery, but it seems likely it was to do with teleportation. Even at the beginning of HL2, the Combine only just managed to get intradimensional teleportation working, and then only with the help of Judith Mossman. It seems unlikely that they had this technology at the beginning of HL1. Other than possible research into technology, the Combine seemed to be mass manufactring alien grunts.
I'm quickly going to propose a theory here: What happened on Xen could have happened on Earth. That is, you might equate enslaved vortigaunts with stalkers, grunt as supersoldiers and nihilanth as Breen. If you look at the alien grunts from HL1 closely, you'll not many similar anatomical similarities to vortigaunts, such as a third arm, siphon like structures on either side of the neck and similar eye positioning. They're just much bigger. This could be a natural evolutionary divergence from a common ancestor, but the prescence of the Combine make me suspect their direct involvement. I believe that the grunts were modified vortigaunts who were designed to be much better than normal vortigaunts at combat. This hadn't happened on Earth yet, but I don't think it was very far off. I don't think Breen had got very far along with his transhuman program at all.
Black Mesa
We still have no idea what happened at Black Mesa, and I don't think we'll ever get much more information. What we do know is that for some reason is that the experiment that Gordon Freeman was ordered to perform seems to have allowed for a massive invasion of aliens to teleport onto Earth.
This was, however, not the first time Earth and Xen met. It seems Black Mesa research teams had been making excursions into Xen for some months before the accident. Research teams had discovered the Border World, Xen, while experimenting with teleportation in the advanced Lambda bunker. Not only did they likely do research on teleportation, but they also did seem to have taken "biological samples", native Xen wildlife, and brought it back to Earth. We can see evidence of this in Blue Shift, were you visit an abandoned human research post on Xen, as well as things scientists say in the Lambda bunker. It's possible that it was these excursions which first made the Combine aware of the existence of a culture with an advanced understanding of the principles involved in teleportation.