The original UFO: Enemy Unknown was built on a simulation-heavy foundation. Players managed everything from individual "Time Units" (TU) for every soldier action to complex inventory management. The sense of war was palpable; losing a soldier was a common, brutal occurrence, but replacements were plentiful.

At its core, XCOM is about "finding certainty within the unpredictable". A Deep Dive Into XCOM and XCOM 2 - Game Developer

The franchise is split between the "classic" era by Julian Gollop and the modern reboot era by Firaxis Games.

Firaxis's XCOM: Enemy Unknown moved toward a "board game" design philosophy. It replaced TUs with a two-action system (move and shoot) and prioritized the "cover" mechanic, making the battlefield easier to read but every loss feel far more tragic and personal. A Study in Tension and Loss