The Extraterrestrial Combat Unit, or X-Com for short, is a top-secret military organization dedicated to protecting humanity from alien invasion. The X-Com is crewed by the most capable soldiers, xeno scientists and engineers from around the world and has access to the most advanced technologies known to man (alien technology included). They have served in at least four alien conflicts, including the three Alien Wars and an encounter with the iconic Sectoids prior to the Third Alien War.
X-Com was formed in the late twentieth century after the discovery of hostile alien activity and numerous failed counterattacks by national militaries. The United Nations called together the most powerful global military powers and formed the X-Com to counter the alien invasion. The following conflict lasted for several years, with the X-Com battling alien forces across the globe, ultimately leading up to the final assault on the alien's main stronghold at the Cydonia region on Mars. The defeat of the aliens and their leader, the Alien Brain, marked the end of the First Alien War, as all alien forces were neutralized after the loss of the Alien Brain. With humanity safe once again, X-Com was disbanded and the spoils of precious alien technology were seized by leading global powers.
Forty years after the Fall of Cydonia, the horrors of the Alien War have been lost to time, and life went on as if it had never happened. The world governments have changed (save for the United States of America), with factions like the Euro Syndicate governing lands once divided by nations. However, a new threat has arisen, unbeknownst to humanity. A new alien army has arisen from the depths of the ocean, a force that has slept beneath the waves for many millennia. Strange alien subs strike from the depths, sinking commercial liners and planes. The alarming rate of the ships lost and the discovery of the new alien threat had led to the leading governments to reinstate the X-Com to combat this new threat. The reformed X-Com was predominantly aquatic, seeing as many conflicts were based underwater save for terror attacks and ship raids. X-Com had fought a desperate struggle, facing incredible and terrifying aliens, some of which were gene-banked prehistoric earth creatures. During the final days of the war, X-Com had finally discovered the alien's main base: a huge underwater city, T'leth, resting at the bottom of the Sigsbee Deep, an impossibly deep area in the ocean located at the Gulf of Mexico. A strike team of seasoned X-Com Aquanauts infiltrated T'leth and eliminated the alien's ruler, a nightmarish beast of tremendous power. The alien armies were destroyed upon its death, however, their victory was bittersweet, as T'leth had risen to the surface and exploded as the "Alien Horror" made a last bid to save itself. The explosion had released a cloud of poisonous gases that subsequently killed off most of the populations of the US, Mexico and Central and South America.
Fifty years had passed after the end of the Second Alien War, and the world has taken a turn for the worse. T'leth's destruction and the toxic gas clouds it spawned had caused a biosphere collapse that would inevitably sterilize the planet. In a last ditch attempt to save humanity, the remaining global powers had created a handful of super-metropolitan cities, for they would allow them to survive the blight that was killing the planet. Expeditions to Mars established mines that carried back enough precious Elerium to facilitate the construction of the new cities, starting with Mega-Primus, built upon the ruins of Toronto.
However, just as fate would have it, there was trouble in paradise. Dimensional Gates appeared out of nowhere above Mega-Primus, and from these portals, a horde of aliens descended upon the city. These aliens were notably different from those faced in the previous wars, bizarre beyond humanity's standards. Once again, the X-Com was resurrected, this time as a security force rather than a military unit. X-Com agents, ranging from humans to human-Sectoid Hybrids to robotic androids, battled against this strange new alien threat in the streets of the city's many districts. Their conflicts eventually reached the alien's home beyond the Dimensional Gates, where they would strike at the heart of their operations.