The Chitauri were a synthetic race living in an unknown part of outer space, were under the control of the warlord Thanos and his servant the Other. The Chitauri operated from a command center, and worked closely with Leviathans.
Loki Laufeyson made a deal with Thanos that he would deliver him the Tesseract in exchange for the command over the Chitauri army, and used the Tesseract to create a portal between Earth and the Chitauri homeworld.
In response to Loki's attempts at conquering Earth, S.H.I.E.L.D. started the Avenger Initiative, a team of superheroes that consisted of Iron Man, the Hulk, Thor, Captain America, Black Widow and Hawkeye. The Avengers faced off against the Chitauri and Leviathan army in New York City, before the Black Widow and Erik Selvig started closing the portal created by the Tesseract and Iron Man sent a nuclear rocket towards the Chitauri command center, causing the remaining Chitauri and Leviathans to be deactivated.