The Unification War is a major fictional event in the backstory of Joss Whedon's cancelled television show, Firefly and its motion picture sequel, Serenity.

Set 500 years in the future, humans have colonized and terraformed a planetary system of unknown location. The system contains "core" planets and "border" planets. As a general rule, the farther out one travels, the less "civilized" society becomes. The core planets eventually allied under a single government, The Alliance, and declared war upon those planets that wished to maintain their independence (The Independent Faction, often referred to as "Browncoats"). This series of events is recalled as the Unification War. The Alliance was victorious, and those surviving independents migrated toward the outer planets.