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.