of the synthetic machines are the insect-like gunships; and Striders, armored creatures that walk on three legs, armed with a high powered cannon and head-mounted pulse turret. In Episode Two, the Hunter, a smaller equivalent to the Strider, is introduced. These tripodal assault machines fire explosive flechettes at targets and are small enough to maneuver indoors. Other synths are seen near the end of the Half-Life 2, though their roles are not elaborated on. Sentry turrets are also used by the Combine. The Combine often uses headcrabs as a method of bioterrorism against dissidents and refugees, firing artillery shells loaded with the creatures into areas and allowing them to infest said area. Appearances Half-Life series According to the backstory presented during Half-Life 2, the Combine appear on Earth after the death of the Nihilanth, the boss character at the end of Half-Life. The death of the Nihilanth, a powerful creature controlling the dimensional rip between Xen and the Black Mesa Research Facility on Earth, causes the rip to worsen, resulting in "portal storms" which spread the hostile wildlife of Xen across Earth. The Combine manipulate this tear in the spacetime continuum, widening it to allow access to Earth from their dimension. When sufficiently wide enough, the Combine launched an invasion in force. Earth is rapidly defeated in a war lasting seven hours (the title is a reference to the Soviet military exercise Щит-82; "Seven-Hour Nuclear War"). Earth's surrender is negotiated by Wallace Breen, administrator of the Black Mesa Research Facility at the time who discovered a means of communicating with the Combine, who is then made the Combine's puppet ruler of Earth, with City 17 as his base of operations. The Combine's first appearance is in Half-Life 2. Through the early stages of the game, Combine Civil Protection units pursue Gordon Freeman through City 17 after his presence is revealed to Wallace Breen. Due to Freeman's actions in Half-Life, Breen sees Freeman as a major threat. As Freeman flees the city, Civil Protection units raid the resistance base of Black Mesa East and capture resistance leader Eli Vance, who is transferred to holding facilities at Nova Prospekt. Freeman and Vance's daughter Alyx break into the facility to rescue him, but Eli is teleported to the Combine Citadel by double agent Judith Mossman. The strike against Nova Prospekt prompts a revolution by the citizens of Earth and heavy street fighting takes