Half-Life: Opposing Force

Half-Life: Opposing Force is an expansion pack for Half-Life following Corporal Adrian Shephard, a HECU soldier in the midst of the Black Mesa incident, later into the story than Gordon Freeman. Opposing Force adds new enemies and weapons to the series, including the mysterious Race-X aliens and the barnacle, as a weapon.

The name "Opposing Force" is mainly a reference to the military term but may also refer to Isaac Newton's "equal and opposite force" law.