Half-Life 2 is set aproximately two decades after the events of Half-Life 1, in which Gordon Freeman was party to an experiment involving transporter technology which led to a 'portal storm' - the opening of thousands of portals between the Black Mesa complex and and an alien world called Xen. Eventually (as covered by Half-Life 1's various expansion packs), Black Mesa was destroyed and the 'portal storm' spread across much Earth's surface. Taking advantage of this rip between dimensions, the Combine - an alien empire - invades Earth. The war between humanity and the combine lasted aproximately seven hours, after which mankind was totally subjugated, and Earth was turned into, for all intents and purposes, a police state controlled by the combine.

Half-Life 2 Review