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What we saw in HL1 was the very end of a long struggle between the Combine and the last of the Nihilanth's race.
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"I'm pretty sure Doug was restating something I'd told him I clarifying it, since it's the foundation on which the series continues. When contacted, Marc Laidlaw provided more information: The Nihilanth and the Combine are now established as former enemies who had knowledge of each other. But the exact nature of the threat was left to be solved in Half-Life 2." We knew that some immense threat had chased the Nihilanth and its creatures out of their own world and into Xen, from which location they were all too glad to seize the opportunity to continue on to Earth with suppression through the Citadels. In other words, we knew that once you cleared out the Nihilanth, you were going to discover something worse beyond it. "We had a glimpse of the larger threat when we were working on Half-Life. The defeated Nihilanth sought refuge in Xen, where the Combine could not (or would not) follow. However, Valve's marketing director Doug Lombardi stated that the Combine had, in fact, invaded a different homeworld from which Nihilanth's race was forced to escape. It was initially speculated that the Combine possibly controlled Xen during the first Half-Life game, based on the similarities to some practices and technologies used by the Nihilanth. Breen refers to the Combine Soldiers as The Transhuman Arm of the Combine Overwatch (However, all human Overwatch units excluding Civil Protection fall under this designation as well) in Nova Prospekt, verifying that "Combine" is an accepted name of the empire, even by the empire itself.
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The name is pronounced COM-bine, as in combine harvester, as opposed to the verb form com-BINE, as in "to join together or unite." Dr. When pronouncing the word "Combine", vocal stress is placed on the first syllable. The reasons behind the Combine's imperialism including their ultimate goal, remain unknown throughout the series. This process results in a highly mobile, aggressive, and adaptive military force which is able to respond to any threat and eliminate any opposition posed by a planet's inhabitants. By manipulating these species through methods including bioengineering and implantation, the Combine creates races of physically superior super-soldiers uniquely adapted for the particular environments of the conquered worlds. The Combine empire expands by invading worlds and exploiting their resources and species as it sees fit. The Combine empire spans multiple parallel universes, is inhabited by an unknown number of sentient species, and is possibly governed by a race of artificially evolved Advisors of the Combine Overworld.
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