Counter-Strike began as a modification for which video game?

Counter-Strike began as a modification for Half-Life.

Canadian game developers Minh Le and Jess Cliffe released the first beta of Counter-Strike in 1999 as a user-made modification for Valve’s first-person shooter Half-Life. The mod reused Half-Life’s engine, characters, weapons, and multiplayer technology while adding a round-based battle between terrorists and counter-terrorists.

The modification became popular through online play and successive beta releases. Valve later hired the creators and acquired the intellectual property, turning Counter-Strike into a commercial game rather than an independent fan project. This history is why Counter-Strike is often described as one of the most influential total-conversion mods.

Half-Life is sometimes confused with the later games that powered Counter-Strike releases. Counter-Strike 1.6 used the original GoldSrc technology, while Counter-Strike: Source moved to Valve’s Source engine and Counter-Strike 2 uses Source 2.