This is the premise of Firaxis Games’ XCOM: Enemy unknown.
Soon, full invasion would begin and the XCOM unit would be Earth’s final hope. To counter this threat, the most powerful nations of the world commissioned the XCOM (E xtraterrestrial Combat Unit) in order to combat the alien threat. It was small strikes at first, but it was clear that humanity was not alone. Then one day, in the near future, the extraterrestrial threat decided to attack. No one believed them, no one believed that there was a threat. In darkest corners of rural Earth, cattle mutilations and little grey men have been sighted by people of arguable credibility.
