United States Government (Half-Life)

The United States Government are the unseen antagonists of the original Half-Life, as they were involved in trying to have the Black Mesa Incident covered up by sending in the Hazardous Environment Combat Unit and Black Operatives to kill any witnesses.

Half-Life
Following the resonance cascade in which alien beings had began to infiltrate the facility under the orders of their leader Nihilanth, the United States Government planned to cover up the incident by sending in the military to kill the hostile aliens, as well as those employed at the facility who were witnesses to the incident. Black Mesa employees initially believed the military were there to help them, until it became clear it was a cover up operation as soldiers began massacring scientists and security guards. One of the Black Mesa employees, Gordon Freeman, began to fight back against the soldiers and killed any who stood in his way.

As the soldiers were failing in their mission, the government sent in a highly trained black operations unit to kill the employees as well as the soldiers themselves, with the endgame being to nuke the entire facility. Black Mesa was later nuked, although several survivors had managed to escape.

Aftermath
During the Combine's invasion of earth, the United States Government, along with the other governments of earth, were overthrown.