As the Portal Storms between Xen and the Earth raged across the planet, spreading Xen life forms across Earth, this caused the planet's populace to move into cities for protection. By the time of the Combine invasion, most governments on Earth were weakened by the Portal Storms.
Attracted to Earth by the dimensional rift caused by the Black Mesa Incident, the Combine, a vast alien empire, launched an invasive force that overpowered the planet by using the now conquered Xen, a crossroads of different dimensions and its Portal Storms on Earth. They caught the human population by surprise and defeating all of its armed forces in only seven hours.
