Left Behind mod for Half-Life 2

The year is 2000. A fatal accident occurs in a test chamber in a Nevada research facility. Nothing is usual about this accident. Aliens arrive in through the Resonance Cascade and quickly overrun the scientists and their security. The US Army has been dispatched to clean it up and make sure word never leaves the facility. Everything went wrong, they miscalculated one scientist... Black Mesa was eventually destroyed, but it was too late, portals began opening around the globe. People move into the cities, protected by soldiers and barbed wire fences, to escape the monsters. The outside of the major cities are barren wastelands that are very inhabitable for human life, due to the aliens. The armies of the world begin a slow task of cleansing the Xen fauna from Earth. It was a costly and long process. People continue their normal lives, protected by the soldiers. In the meanwhile, the Combine are preparing an invasion force, to conquer Earth in a quick surgical strike.