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. The United Nations Secretariat Building in New York City is destroyed and Earth is rapidly defeated in what became known as the Seven Hour War (the title is a reference to the Soviet military exercise Щит-82; "Seven-Hour Nuclear War"). Earth's surrender is negotiated by Wallace Breen, who is then made the Combine's puppet administrator of Earth, and uses City 17 as his base of operations.