There seems to be a lot of confusion about what has been hapening in Half-Life 1 and Half-Life 2.Therefor i have written an overview of all events that transpired during the Half-Life Saga.
Here are my thoughts on what's been going on so far in the Half-Life saga. I'm assuming you've beaten Half-Life 1 and Half-Life 2 as this spoils everything.It took me around 2 weeks to fully complete this as i had to verify everything.Enjoy,i'm sure you will find this most interesting.
1995-The Combine Invades Xen
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. In Earth's case a few years down the road, the leader is Wallace Breen.
Back on Earth, Gordon is studying for his doctorate at MIT. At Black Mesa, small 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.
2000-The Black Mesa Incident - (Half-Life 1)
The Resonance Cascade - The event that started it all.This event caused the fall of the entire human race. This cascade was caused by some special samples, which Breen selected. How Breen was involved and how much he knew could and would happen with the resonance cascade is simply not known.
Gordon's Sponsor - In the Half Life 1 opening sequence, the computer reads "Sponsor: Classified " This mysterious sponsor is most likely G-man, who has forseen the events of the future and is basically guiding Gordon into that test chamber.
Black Mesa is Gordon's first "unofficial" assignment. G-Man made sure it was Gordon with the HEV suit, made sure it was Gordon that was the only one that could survive the radiation. Gordon's entrance exam was Black Mesa.
Gordon Freeman kills Nihilanth, freeing the Xen from the Combine and alerting the Combine of Earth. This Xenian was the leader of the Vortigaunts, and now the Vortigaunts are free. But now, however, the Combine have discovered Gordon Freeman's presence on Xen and the portal he came through. The Combine discover the existence of Earth and plan an invasion of it.
At the end you must realize that the portals between Xen and earth did not close at the end of Half-Life 1, they are still open.
Gordon is Hired - At the end of Half-Life 1 G-Man gives Gordon a choice between his death or working for the G-Man. Half-Life 2 assumes you 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.
2002-The Portal Storms---------Gordon not aging!!!!!
Black Mesa has a huge ripple effect on the rest of the world after the events of Half-Life 1. 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.
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.
2004-The Combine Invades Earth / The Seven Hour War---------Gordon not aging!!!!!
Using the Xen-Earth portals as transport gateways, The Combine launches a multi-pronged, all-out assault on the entire Earth. The Combine uses the same techniques 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 Xenian monsters who are not under the control of the Combine anymore. 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.
In a war lasting only seven hours, the Combine annihilates Earth. Dr. Wallace Breen, the old Administrator of Black Mesa (note: the Administrator in Half Life 1 is not G-Man) arranges and manages Earth's formal surrender to the Combine and in return for his actions the Combine allow him to become the Administrator of the Overwatch. Therefore, Breen is made the species-specific leader (Like Nihilanth in Xen). This helps propagate the lie that the enslavement of humans is a good thing. 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.
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.
evidence]
2004-Rise of the Combine Overwatch---------Gordon not aging!!!!!
Wallace Breen takes residence in its top floor of the City 17 Citadel, attempting to quell the rebels. Part of his plan is to also persue a psychological game with the remaining citizens of Earth, assuring them that the Combine takeover is in the humans' best interests. In fact, we have reason to believe that he actually might believe his own lies - he is so deranged with his contact with the Combine that he actually believes that the conquest of Earth is in humanity's best interest. After the Portal Storms, the humans are in such disorder that they will believe something like this. Ruling by fear, the Overwatch assure to continue to protect human city centres from the outside aliens, and thus fearful humans continue to live in the cities.
Why does Breen betray humanity? One can only assume that Wallace Breen understands that the Combine will enslave all of humanity one way or another. In order to save his own life he surrenders all of Earth in exchange for the sparing of his life. Thus, he brokers a deal with the Combine: if he can deliver Earth to the Combine Overlord, he can be transported safely away from Earth in a Combine portal. Until then, it is up to him to quell the rebels using the Combine-Hybrid forces until Earth is suitable for Combine inhabitation. He fuels his work with the mental construct that what he is doing is in the best interest for humanity.
The Combine begin some transitional tasks. First of all, they begin to replace all the air on Earth with a new type of air, one which the humans can hardly breathe. They do this through the Air Exchange Program, a key element to the Combine Overwatch. Second of all, the Overwatch enact a Supression Field to prevent humans from procreating. They do this so no new humans can be born, and that the humans left on Earth will be the final generation of them. They also put something in the water supply to make people gradually forget the past, and so they can more easily comply with Breen and the Combine. Finally, the Combine begin to drain Earth's oceans. These tasks show that the Combine are are preparing Earth for new residents.
2010-The Rebellion - (Half Life 2)---------Gordon wakes up!!!!!
The Human Rebellion is Gordon's first official assignment, and his second assignment under the supervision of G-Man. G-Man pulls Gordon out of the slow-time warp for his assignment. Once Gordon is discovered, Wallace Breen makes his capture or death a top priority for Combine Overwatch officers.
Gordon continues to slip out of the Combine's grip and at Nova Prospekt, lands a huge blow against the Combine. This sparks a massive revolution among citizens, who begin a much larger rebellion than before. Chaos erupts in City 17 as Wallace Breen and the Combine Overwatch try frantically to end the spreading revolution, and the Citadel is put into full attack mode, with Combine soldiers flooding the streets. Vortigaunt refugees from Xen ally with the resistance, as they see the Combine as the greater threat. G-Man oversees Gordon's adventure,
assisting him where necessary.
2010-The Fall of the Combine Overwatch---------Gordon is put back into stasis!!!!!
At the end of Half Life 2 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 assignent 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, 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?
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 Half Life 1.This rebel force has incredible technology to be able to control time in the way G-Man does. G-Man lures the Combine to Earth by causing the resonance cascade. Perhaps he wants to use Earth as a battleground to distract the Combine from events we are not aware of. 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.
Pity my fps dies on 32 player servers, at one point it even went down to 10fps
I need to get my butt in gear and get some of her gifts under there. 
