With an inexplicable display of supernatural forces, LA (Los Angeles) was brought under siege. The only thing people saw on the news was aerial shots of the city being destroyed by monsters - demons, vampires, etc. The nation sent an army, the Slayer Organization sent squads to the city,and Ugenta vampires volunteered themselves to fight for humanity. There were many casualties but eventually the battle was won. However, the public was made aware of the creatures that go "bump in the night," which started The Great Revelation. LA was eventually stabilized but uninhabitable.
Currently, it is closed off by the government, perimeters gaurded by the Initiative and other military operatives. Even the armies' forces can't handle all of the things lingering through the streets of the once populated city. New monsters, creatures, and bad supernatural energies come from that place.
Here is what the public didn't see on the news. Angel, an animinus vampire with a soul (very rare), took over the LA branch of the evil law firm Wolfram & Hart as CEO - an attempt to defeat the evil from the inside out. Angel and his team found it a difficult task, as they tried to prevent themselves from being corrupted as well. In the end, Angel set in motion a plan to destroy the Circle of the Black Thorn, a secret council of demons that serve as the Senior Partner's (the head demons in charge) earthly representation. His plan succeeds, but as retaliation the Senior Partners unleash "hell" (literally) on LA. Vampires, demons, and other creatures raged war on Angel, his team, and LA.