XCOM: Enemy Unknown tasks you with defending Earth against hordes of invading aliens, commanding a force of soldiers putting their lives on the line to push back the threat. That's no joke: If one of the commandos under your watch dies, he stays dead, taking his hard-won experience with him. Too many wrong moves could leave your squad stacked with rookies rather than grizzled vets.
XCOM's tactical, turn-based combat is tough, but the game gives you plenty of time to think through your moves. Between missions, you deal with organizational tasks—managing finances, expanding XCOM operations, researching newly uncovered alien tech, et cetera. The single-player game is plenty long, but the enemy placement in battle is randomized, so every play-through is a unique experience.
XCOM 2 ratchets the tension even higher by putting you on the offense, as XCOM becomes a guerilla force in a world conquered by aliens. The core gameplay's largely the same, but during the tactical phase you have to balance between striking the aliens where it hurts while avoiding their counterattacks, juggling scarce resources all the while. It's excellent.