Chitauri

"The Chitauri grow restless." "Let them gird themselves. I will lead them into glorious battle." "Battle? Against the meager might of Earth?" "Glorious, not lengthy. If your force is as formidable as you claim."

- The Other and Loki

The Chitauri are a cybernetic alien race governed by a hive mind and subservient to the overlord Thanos. They were led by the Asgardian Loki during the War for Earth but were defeated by the Avengers.

History
"He has an army called the Chitauri, that none on Asgard nor any world know."

- Thor

When the warlord Thanos allied himself with the exiled Loki, the Chitauri were granted to him as his personal army to conquer Earth and gain the Tesseract.

Once having gained the Tesseract from S.H.I.E.L.D., Loki was able to open a portal to the Chitauri fleet standing by, thanks to the help of a brainwashed Erik Selvig. The Chitauri immediately started to wreck havoc on New York City, destroying buildings and gunning down civilians in the streets. The National Guard could not deploy enough troops quickly enough to counter the invasion. Luckily, the Avengers arrived, having fought Loki repeatedly in an attempt to hinder his efforts at summoning the Chitauri. Soon after their arrival, a major battle ensued between them and the Chitauri.

Not expecting such a resistance, the Chitauri were initially overwhelmed, but soon their numbers increased so much that the superheroes found some difficulties in containing them. Captain America took command of the Avengers as well as the police forces, and thanks to his command, the Avengers' powers were properly used while they fought the menace. Eventually, the World Security Council attempted to destroy Manhattan with a nuclear bomb in order to contain the Chitauri invasion, but Iron Man intercepted the bomb and disposed of it into the portal, where it impacted and obliterated the Chitauri Command Center, instantaneously disabling all Chitauri forces on Earth, thus ending the invasion.

After their defeat, pieces of Chitauri technology were recovered by both S.H.I.E.L.D. and various other parties.

A Chitauri was one of the specimens being held in Taneleer Tivan's museum before its explosion.

Characteristic Traits
The Chitauri are a powerful, reptilian warrior race, a hybrid between organic beings and machines. They have a caste-like society, with each caste (nearly a different species in itself) fulfilling a different role in Chitauri society. They're controlled by a Mother Ship which never shows on the battlefield, and act as one during war. They have superhuman physical attributes, superior strategy, energy-based rifles and flying vehicles, but their main strength is their gigantic amount of soldiers in combat.

Allies

 * Thanos - Master
 * Loki - Commander
 * Leviathan

Enemies

 * Avengers
 * Steve Rogers/Captain America
 * Tony Stark/Iron Man
 * Bruce Banner/Hulk
 * Thor
 * Natasha Romanoff/Black Widow
 * Clint Barton/Hawkeye
 * Beth - Hostage

Trivia

 * In the comics, the Chitauri are a shapeshifting race kin to the Skrulls, while in the movie they appear more like cyborg-like creatures. They were one of the first threats faced by the Ultimates, but their background story reveals that they were present on Earth even during World War II.
 * The name "Chitauri" derives from, the name of a binary star in the constellation.
 * The Marvel Cinematic Universe variation of the Chitauri is similar to the Phalanx, a cybernetic alien race from the comics who communicated through a hive minded intelligence.

Behind the Scenes

 * Joss Whedon revealed that he chose to include the Chitauri over Skrulls or Kree because this particular race didn't have any wide background story and they could be used even for a small part.