Firaxis’ 2012 strategy title was a remake of a 1994 original, UFO: Enemy Unknown, which cast you as the commander charged with responding to an alien invasion of Earth – building up the eponymous organisation, responding to attacks across the globe, and hunting down alien operating bases.
A mong XCOM’s distinctions is that it is a difficult game, so the sequel’s concept is quite brilliant: the first time around, you lost.