The Combine Empire invades and takes control of Xen, eventually turning the entire planet into a slave race. The Combine uses its usual technique to destroy the Xenians. That is, they hybridize the dominant species of the planet (In Xen's case, the Vortigaunts) with their own technology to create a "Super soldier" that is adapted to Xen's environment and uses powerful Combine technology. This soldier can effectively take over the planet. The Combine also bribe a member of the victim species to act as the "leader" of the planet to encourage other members of the species to conform to Combine demands. In Xen's case, this leader is the Nihilanth (although it is unknown if Nihilanth is a Vortigaunt or not). It is interesting to note that unlike Breen, Nihilianth is forced into complying with the Combine. Perhaps Nihilanth was the leader of Xen before the Combine invaded Xen, and the Combine had to convert him by other means.

Back on Earth, Gordon is studying for his doctorate at MIT. At Black Mesa in New Mexico, teleportation experiments are taking place. Under the supervision of researchers Eli Vance and Dr.Kleiner, crystals are retrieved from Xen, and organisms are analyzed. Research is developing.

Back on Xen, the Combine have discovered Black Mesa's portal from Earth. They contact Wallace Breen (how they do this is not known at the time), the Administrator of Black Mesa. Promising him power and riches, the Combine persuades Breen to allow the Combine to bring Earth into its "Universal Union", convincing him that this is humanity's greatest opportunity to see the stars, and embrace the universe. Breen takes the bait, believing the Combine truly want to help humanity (and not enslave them, as they eventually do). The Combine prepares to take Earth.

They're waiting for you Gordon, in the test chamber.

(i) Breen Triggers the Resonance Cascade - Wallace Breen, under supervision by the Combine Overlords, triggers a resonance cascade within the Anomalous Materials sector of the Black Mesa complex, by selecting a specific test sample to be used. His selection of this hazardous material is masked by a system crash, which Breen most likely triggers to cut out formal communication between scientists. G-Man is aware of Breen's plan and tries to prevent the Resonance Cascade, but fails. This "volatile portal" allows the Combine to send their initial force, the Xen slaves, to take over Black Mesa and prepare for the main invasion. Of course headcrabs and other Xen wildlife decide to hitch a ride to Earth, for the fun of it. G-Man prepares a task force to deal with the main portal and prepares US soldiers to fight the aliens. Black Mesa becomes a messy situation, as many species and technologies clash.

One man in a HEV suit becomes of great interest to G-Man during the incident, however. Gordon Freeman, a newly assigned scientist in the Anomalous materials lab, shows exceptional skill in dealing with Xen footsoldiers and U.S. troops. G-Man's interest peaks as Freeman navigates through Xen, and defeats the Nihilanth, freeing the Xen slaves, striking a major blow against the Combine. G-Man is impressed.

But before this, Gordon does what he's best at, following instructions. He unknowingly helps the Combine campaign by launching a Lambda satellite into space. This mission-critical satellite is most likely what the Combine need to conduct the Portal storms by opening up portals world-wide.

Black Mesa is destroyed by nuclear detonation (noted in OF), but the Combine have what they need in place - an unstable portal to Earth and a satellite to conduct a portal storm with.

(ii) Gordon is Hired - At the end of Half-Life G-Man, impressed by your work, gives Gordon a choice between his death or working for the G-Man. Half-Life 2 assumes he chose life over death. After you choose to work for him G-Man puts you in a slow-time-warp (i.e., you are put into stasis) to call upon you when needed.

Black Mesa has a huge ripple effect on the rest of the world after the events of Half-Life 1, as the Combine open up portals world-wide, via its portal-network satellite. Aliens (from Xen) of all types are teleported into suburbs. Headcrabs crawl along the aisles of grocery stores while others teleport into swimming pools and homes. Utter chaos erupts on Earth. The freed Vortigaunts flee to Earth to escape the Combine, which are struggling to regain control of them.

A huge population shift occurs as people move into the cities, protected by soldiers and barbed wire fences, to escape the monsters. A feeling of safety comes over the people of Earth, as they can once again live normal lives within the confines of the cities, as the aliens continue to infest the outside world.

Meanwhile, the Combine prepare for an assault on Earth, assured that the required portals are in place. Now that the Xenians have fled to Earth, the Combine can forget about Xen and enslave both Mankind and the Xenians in one fell swoop, on Earth.

Using the established Xen-Earth portal network as transport gateways, The Combine launches a multi-pronged, all-out assault on the entire Earth, teleporting troops as well as drop-ships, gun-ships, and striders from Combine off-world outposts. Once there, the Combine uses the same technique that they used on Xen on the humans, building a super-army out of the combination of the DNA of the most dominant species on Earth (the humans) and Combine technology.

The main Citadel is established in City 17. Other Citadels appear in other cities, seemingly out of thin air. These huge structures are home to Combine control centres. Combine armies are produced here as well as vehicles and armaments.

Also, while fighting each other, the humans and Combine also have the threat of the Xenians. The freed Vortigaunts ally with the rebel forces, wanting revenge against the Combine. The headcrabs, being headcrabs, just attack anyone they can find. Other monsters find their place assaulting both Man and Combine.

The next stage of the Combine's plan takes root. Wallace Breen, still optimistic about the Combine takeover, is made the Administrator of Earth, as promised back in Black Mesa by the Combine. Breen surrenders Earth to the Combine, explaining that the Combine are here for good, to immortalize humanity. Even though he sees the corpses of fellow humans, he believes that the loss of life is inevitable in the transition period, as Earth joins the Universal Union. Breen formally "surrenders" Earth after 7 hours of combat between Mankind and the Combine. Breen being a human helps the Combine, too. He is a species-specific leader (Like Nihilanth in Xen) - this helps propagate the lie that the enslavement of humans is a good thing, and humans, seeing a human in power, can be more readily fed world-changing events that are not in their interest.

Most of the major population centres of Earth are destroyed. People move into the country, spreading out to avoid Combine capture. Small pockets of rebel fighters surface, but have to establish bases in un-safe areas in the country, where the Xen wildlife have manifested themselves, finding niches in Earth's ecosystem. Still under the spell of the Combine, and ignoring the massive genocide, Breen views the building rebel force as a group of short-sighted troublemakers.

(iii) Cities renamed. The remaining populated areas are named City 1, City 2... City 17, City 27, etc and are placed under control of the Combine, who are establishing a huge presence on Earth. The United Nations in New York City is taken over and destroyed. [ screenshot, from Eli's lab ]

At the end of HL2 you complete your assignment, by destroying your target, the Citadel in City 17, the Combine throne on Earth. It is unknown what becomes of the earth-based Combine, but most likely they are left leaderless - with the Citadel destroyed, the Overwatch has no instructions and back-up, and can be dealt with by the rebels. Once the remaining Combine soldiers are killed, the Combine presence on Earth will be depleted. Thus, Gordon's assignment has been completed, and he is put back into a slow-time-warp, awaiting his next assignment.

What does this mean for the Combine? Well, the Combine must either re-take Earth by re-taking City 17's Citadel through the Xen portals, and re-establish leadership and structure, or give up trying to fight Earth. I believe that they will continue to try to take Earth for themselves, a political move that will show other planets and systems that they are truly invincible. One thing is for sure: the Combine are not extinct. They have many more troops on multiple dimensions and planets; their empire is still expanding, ruthlessly invading planets/dimensions like Earth and Xen.

What did Gordon accomplish? He ended the rule of the Combine on Earth (for the time being), and struck his second serious blow against the Combine, showing them that both humans have strength to fight a mighty empire and that the Combine are not all-powerful. Humanity once again is free for the time being.

Why did G-Man give Gordon this assignment? Good question. The way things are going, I believe that G-Man is an agent of an interstellar rebel alliance (assuming a human form), seeking to try to break down the ruthless Combine empire, an empire which I believe existed far before the events of HL1. This rebel force has incredible technology to be able to control teleportation in the way G-Man does. Perhaps he realizes that Gordon Freeman is the only one who can defeat the Combine empire, and keeps him on call for this sort of thing. You have to remember, Gordon also freed a different race: the Xenians. So this is the second big blow Gordon's struck against the Combine. The funny thing is, he doesn't even realize it. I do not believe that G-Man is Gordon Freeman from the future.