First video game to feature a fully stealth-based co-op mode

Spies vs. Mercs was a tough act to follow in 2004, but with multiplayer and co-op proving essential for action games approaching 2005, following the likes of Halo and Gears of War, Ubisoft looked to introduce co-op to Splinter Cell: Chaos Theory. It became the first game to incorporate co-op tactics into a fully-fledged stealth-focused gameplay mode, enlisting two players as Third Echelon Splinter Cell agents in a series of purpose-built standalone missions. In later games, Ubisoft expanded the concept to expand on the main single-player narrative.