Half-Life: Opposing Force is an expansion pack for valve's science fiction FPS Half Life. The game was developed by Gear and Valve corporation and published by Sierra on November 19, 1999. Opposing Force is the first expansion for Half-Life and was first announced in April 1999.

Opposing Force returns to the same setting as Half-Life, but instead portrays the events from the perspective of a US marine, one of the enemy characters in the original game. The player character is sent in to neutralize the Black Mesa Research Facility when a scientific mishap causes it to be invaded by aliens, but quickly finds that the Marines are outnumbered and slowly being beaten back by a second alien race and black operations units.

Opposing Force was received well by critics, many describing it as the new benchmark title for expansion packs, in a similar fashion to how Half-Life revolutionized the first-person shooter genre. Other reviewers, however, thought that the game still had many of the negative aspects of other expansion packs, although it was still regarded as an exceptional add-on.

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