The Uprising

The Combine War took place an unspecified amount of time after the Black Mesa Incident in the original Half-Life. The war was fought between the Combine Empire and Earth's Human Resistance Forces, the latter aided by Vortigaunts.

Prelude
The Black Mesa Incident caused Portal Storms to erupt all across Earth, attracting the attention of the Combine Empire. The Combine's motive for invading Earth is unknown; it is surmised that the Combine simply wished to take control of Earth's resources and enslave its population, as it had done many times before to other planets. The Combine invaded Earth, quickly defeating the planet's armed forces in the Seven Hour War. Dr. Wallace Breen negotiated a surrender on behalf of the surviving human population; he was instated as ruler of Earth, and aided the Combine in rounding up the civilian populace into large, tightly controlled cities. Breen chose City 17 as his de facto capital, and the Citadel was embedded there to house the apparatus of the new government. It is likely that what was left of Earth's armies were assimilated into the Combine's occupational force.

The new rulers of Earth governed with an oppressiveness bordering on the genocidal; indeed, such was the calculated brutality of Combine rule that it seems likely that the alien rulers did intend to run Earth and the human race into the ground, despite Breen's assurances otherwise. Dissenters were turned into Stalkers or simply killed, and a Suppression Field was installed to prevent humans from breeding. This was coupled with a gradual leeching away of the planet's natural resources, notably in the drastic drainage of the Earth's oceans. At the time of Gordon Freeman's entry into City 17, the Combine's army on Earth consisted entirely of processed humans, so-called post-humans and military synths.

The Resistance
In response to the Combine take-over a number of the subjected formed the Resistance, a loose band of guerillas, outlaws and spies dedicated to overthrowing the new world order. Unable to take on the alien empire directly, the Resistance instead worked at undermining it by infiltrating the ranks of Civil Protection, smuggling weaponry, setting up a sophisticated underground with bases at Ravenholm, Black Mesa East and White Forest, and by helping civilians escape the Combine controlled cities. The Combine suppressed the Resistance at every available opportunity, most notably in the case of Ravenholm, which was subjected to a massive headcrab bombardment with only one known survivor. Although the Resistance had some successes, according to Judith Mossman the Combine was largely aware of the Resistance's activities, and only allowed its continued existence because it didn't consider the rebels an appreciable threat.

Half-Life 2
After the reappearance of Gordon Freeman, matters escalated significantly. In a panicked attempt to locate the doctor, who had become a figure of legend amongst the Resistance, Civil Protection began systematically destroying all known Resistance bases in City 17. Despite this Freeman was able to escape the city, and again evaded the Combine after it reacted to intelligence received from then-double agent Mossman and attacked Black Mesa East, capturing Eli Vance. Again, a public broadcast by Breen seen in Nova Prospekt suggests the Combine already knew the location of Black Mesa East, and "could have taken Vance at any time in the past ten years".



The battle shifted against the Combine when Freeman led a huge army of Antlions against the Combine's facilities at Nova Prospekt, in an effort to retrieve Vance. Although Freeman was unable to do so, the Antlions overwhelmed the Combine there and their prototype teleporter was also destroyed in the fighting. The Resistance, who had previously only operated underground, took this as a signal to begin an outright uprising against the Combine, with many citizens joining their ranks. A massive battle ensued in and around City 17 which lasted almost a week, with no side able to gain an advantage over the other. Eventually, with Freeman's assistance, the Resistance forces were able to capture the Combine Overwatch Nexus, the central control centre for operations in the city. As the Resistance encroached upon the Citadel walls, the Combine handed the streets over to Striders, who proceeded to exterminate large numbers of rebels.

This, despite heavy losses on both sides, ultimately proved ineffective; following ferocious fighting in the city centre, the Resistance forces reached the Citadel walls and Freeman was able to infiltrate the Citadel. There, he critically damaged the Dark Energy Reactor at the top of the building, destroying the Earth-bound Combine's link with the rest of its Empire and its seat of power in one fell blow.

Episode One
In Episode One, with the Citadel rapidly falling apart the remaining Combine attempted to reestablish the link with the rest of their Empire by destabilizing the Citadel's Core, in order to send an important message to its leadership. As well as making a copy of the data packet the Combine were planning to send to their leadership, Gordon and Alyx temporarily stabilized the Citadel's collapsing core for long enough to get most, if not all remaining citizens out of City 17. Whilst they and the few elements of Resistance remaining in the city battled their way outwards, the Combine destabilized the core again and successfully sent its message out, at the cost of utterly destroying the Citadel, most of City 17, and presumably anyone who remained in the metropolis.

Episode Two
In Episode Two, the cataclysmic destruction of City 17 is revealed to have caused an ongoing Portal Storm in the heart of the city, a purposeful development manufactured by the Earth-bound Combine to allow them to reconnect with the rest of their empire. With fears that this would bring about a "seven minute war", the Resistance began preparations to launch a satellite to tap in the Xen satellite array used to help bring an end to the Black Mesa Incident, with the hope that this would close off the growing superportal. Upon hearing about the data packet Gordon and Alyx had retrieved from the Citadel, the Resistance leadership realised that the two had stumbled upon the Citadel's unique data signature and coordinates to the Combine Overworld, a vital component for the satellite array. They urged them to reconvene with them at White Forest, where the satellite was being prepared.

They did so only slightly ahead of the Combine, which had rallied its remaining troops in the area and begun the advance upon the Resistance base. With the help of Freeman and the Magnusson Devices the Resistance fought off the determined Combine offensive against White Forest. The satellite was successfully launched, and it succeeded in bringing an end to the superportal, effectively shutting the door on the Combine remaining on Earth.

Having absorbed the revelations contained in Dr. Mossman's message, Gordon and Alyx began preparations to travel north by helicopter and find Mossman and the mysterious Borealis. As they were making their final preparations, however, they came under attack from flying Combine Advisors. Although they were eventually forced to retreat by Dog, they succeeded in killing Eli Vance, one of the most important figures of the Resistance.