Half-Life: Opposing Force

Half-Life: Opposing Force is an expansion pack for the game Half-Life. The game was developed by Gearbox Software and Valve and published by Sierra Entertainment for Windows in 1999. Opposing Force was the first expansion for Half-Life and was announced in April 1999. Lead designer Randy Pitchford believed Gearbox was selected to develop Opposing Force because Valve, the creators of Half-Life, wanted to focus on their future games.

Opposing Force returns to the same setting as Half-Life, but instead portrays the events from the perspective of a U.S. Marine, one of the enemy characters in the original game. The player character, Adrian Shephard, is sent in to neutralize the Black Mesa Research Facility when a incident causes it to be invaded by aliens, but quickly finds that the Marines are outnumbered by enemies.