"Amputate,Malignant,Coagulate,Infection,Dissect and Diagnose"

"Welcome. Welcome to City 17."

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. The Combine empire expands by invading worlds and exploiting their resources and species as it sees fit. 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. 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. The reasons behind the Combine's imperialism including their ultimate goal, remain unknown.

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Civil Protection

Part of the Combine Overwatch, Civil Protection officers are ordinary volunteers who have willingly joined the Combine,either for more privileges, such as additional and higher-quality food, better living conditions, an increase in authority and status over others, or out of genuine sympathy and identification with the Combine's aims. As such, they have not been subject to transhuman modification in any way, unlike other Overwatch units.

Overwatch super elites

The Overwatch Elite are specialized, higher-ranking Overwatch Soldiers. Elites are very tough and more durable than other Overwatch troops, achieving marksmanship accuracy with their weapons. they act as high special commandos attached to other Combine units, and generally reserved for more critical combat missions, where their deadly skills and efficiency are required. They are typically deployed in squads of their own kind, but will operate with other Combine units for support.

The Quarantine Zone

the rift opened by the Black Mesa Incident allowed alien flora and fauna from the Xen borderworld to infest areas of Earth. After their successful conquest of the planet following the Seven Hour War and the worldwide Portal Storms, the Combine established the Quarantine Zone to sequester the Xen infested sections of City 17 and prevent further propagation of alien species.