Half-Life: Opposing Force

Opposing Force is an expansion for Half-Life, developed by Gearbox Software and released in 1999.

Plot
After the resonance cascade disaster, the military initiates a cover-up operation to eliminate all alien presence at Black Mesa, as well as silence any witnesses. You play as marine Adrian Shephard, who is separated from his mates after his transport is downed by an alien aircraft. As the game unfolds, Shephard finds that the government's Black Ops team also intends to eliminate his unit to complete the cover-up.