. "Half-Life: Opposing Force is an expansion pack for Valve Software's science fiction first-person shooter video game Half-Life. The game was developed by Gearbox Software and Valve Corporation and published by Sierra Entertainment on November 19, 1999. Opposing Force is the first expansion for Half-Life and was first announced in April 1999."@en . "Half-Life: Opposing Force"@en . . . . . . . . . . . . .