UFO: Enemy Unknown (later released under the title X-COM: UFO Defense in the United States and X-COM: Enemy Unknown for the European market) is a video game created by Julian Gollop and published by MicroProse Software in 1993. It is the first game in the X-COM series.

The story of X-COM begins in 1998. The initial plot centers around increased reports of UFO sightings. Tales of abduction and terrorism by the unknown aliens become widespread. The nations of the world come to perceive this as a threat and attempt to form their own forces to deal with this, such as Japan’s Kiryu-Kai; these forces fail miserably, the Kiryu-Kai not intercepting a single UFO in its five months of operation. On December 11, 1998, representatives from some of the most powerful nations in the world meet in Geneva, Switzerland, to discuss the issue. From this meeting was born the Extraterrestrial Combat Unit – X-COM, which the player takes control of during the course of the game.

[Above lines from Wikipedia : http://en.wikipedia.org/wiki/UFO:_Enemy_Unknown ]

Note: The Original X-COM: UFO Defense Version 1.4 required!