After Hell's catastrophic invasion of Earth in the Earth-Hell War, the United States took steps to prevent such an invasion from recurring. The old UAC corporation was refounded, under completely new management (because the old trustees and stockholders were all dead, this wasn't much of a problem), and sent to research tools and technologies to prevent such an incursion from happening ever again.
Though the invasion had been stopped, and the remaining demons were gradually being exterminated by mopping-up squads, it was clear that the powers of Hell remained strong. While the Spider Mastermind and Baphomet seemed to no longer threaten, who knew what else lay outside? Waiting. Watching. Preparing.
The new UAC began working on Quantum Accelerator Devices, intended to close interdimensional Gates at a distance and so prevent future incursions forever. The project began innocently enough. Naturally the scientists, in order to learn how to close Gates, had to relearn Gate technology first. This ability was rapidly regained. Perhaps too rapidly.
The Terror Edit
Soon, beings from Outside had their dire attention drawn to the new experiments, and then, one day, a Gate opened in the heart of the research complex. Unnatural horrors from the pit poured in, ravening for destruction. But the UAC scientists had learned their trade. The Quantum Accelerator Device performed perfectly in its maiden test -- the invasion Gate was closed instantly and permanently when the Accelerator flicked on. A cyberdemon, halfway through, was snipped in two when the Gate closed. Earth would now be safe from literal invasion by Hell. At least, once the technology could be set up around the globe.
The next day, a ring of seven Gates opened throughout the base, and a monstrous legion rampaged through. The Quantum Accelerator began putting out the Gates at once, and within an hour, six were closed. But the hellish army was now too strong, too numerous. The marines fought like mad dogs, but were finally pulled down by the enemies' claws. The scientists, marines, and bureucrats were all slain or transformed into undead mankillers.
The Task Edit
The Quantum Accelerator and its prototypes were deep inside the ravaged complex. A demon Gatekeeper guarded them and manned the last Gate of Hell. The government, frantic that the Quantum Accelerator would be destroyed or used in some alien fashion upon them, ordered all marines to the site at once, regardless of their location.