The Chitauri Invasion was one of the first wars between humanity and beings from other worlds. The war lasted for less than a week, but the impact it left was great.
Taking place on Earth, the war was fought by the exiled Asgardian prince Loki and his Chitauri allies against the applicants of the Avengers Initiative and the espionage agency known as S.H.I.E.L.D.
At first the members of the team refused to cooperate with each other, but after Phil Coulson's demise at the hands of Loki, they were able to work together and defeated the Chitauri at the Battle of New York.
The events of the war were set in motion shortly after the Battle of Puente Antiguo, when a fight between Thor and Loki in Asgard resulted in the destruction of the Bifrost Bridge. This caused a temporal anomaly that swallowed up Loki, teleporting him to a region of space virtually unknown to the Asgardians.
Here, he met Thanos. Through interrogation, Thanos learned of the history regarding Loki's people, but the mention of the Tesseract was what truly piqued his interest. The Tesseract was, in fact, one of the Infinity Stones, a collection of powerful objects that Thanos had been seeking in a mad quest to wipe out half of all life in the cosmos in an attempt to create a measure of universal balance.
Seeing a chance for greatness, Loki brokered a deal with Thanos, offering to give him the Tesseract, which was being studied by S.H.I.E.L.D. on Earth, in exchange for an army to conquer the Earth.
Initially sceptical, Thanos agreed and granted Loki leadership over the Chitauri, a powerful, technologically advanced alien race.
Over the course of the following year, Loki planned out his invasion, controlling scientist Erik Selvig from afar and using him as a vessel to study Earth and the threat its inhabitants may pose.
In the year 2012, Loki was finally ready to begin his invasion, and was given a Scepter by Thanos. The scepter contained another Infinity Stone that had the ability to control the minds of others, and would aid Loki greatly in his mission.
Then, through manipulation of cosmic energy, Thanos teleported Loki to Earth.