X-COM

The X-COM series began in 1994 with UFO: Enemy Unknown, created by Mythos Games and published by MicroProse for PC/DOS. The game combined strategic base management, research and funding with turn-based tactical battles against an alien invasion, in which every soldier could die permanently. This mix of permanent loss, tension from an unknown enemy, and the need to plan every move carefully became the defining trait of the series and shaped the turn-based tactics genre for decades. The sequels Terror from the Deep and Apocalypse expanded the mechanics further, but the core principle - defending Earth against an alien threat through the secret organization X-COM - stayed the same.

After a few weaker action spin-offs in the late nineties, the series went quiet for years until Firaxis Games revived it in 2012 with XCOM: Enemy Unknown for modern PC. The new version kept the core - turn-based combat, fear of losing units, base management - but made it more accessible with clearer controls and faster pacing. The reboot's success was confirmed by XCOM 2, which raised the difficulty and tactical depth further. The 2012 reboot is now often seen as the series' high point for how elegantly it merged classic principles with modern design, and together with the original UFO: Enemy Unknown it strongly shaped the wave of turn-based tactics games that followed.