The Chitauri are a nomadic alien race. They appear in having flesh-coloured, bulbous, tentacled, multi-pedal forms. They are a conquering race that had sought for a suitable, fertile planet.
The Chitauri are governed by the Chitauri Federation. An ancient, nomadic race, the Chitauri had for untold years been searching for a suitable homeworld. In each world they occupied, they drain its resources and then move on to the next planet, and thus repeating the cycle. The Chitauri Federation had in the past clashed numerous occasions with the people of Krypton and have been utterly defeated every time. Because of these defeats, the Chitauri loathed the Kryptonian race that they even used the word "Krypton" as a profanity.
Sometime in the 1930s-1940s, the Chitauri arrived in the Solar System and invaded Mars. The Chitauri waged a larger campaign on the red planet while a smaller military detachment expanded to Earth during the Second World War. On Earth, the Chitauri, led by one of their species under the alias Herr Kleiser, infiltrated the planet among the human population and allied with Nazi Germany, in which they provided their expertise in the Nazi nuclear program in an effort to wipe out humankind. By 1945, they succeeded in creating a nuclear missile and had it aimed at the United States; however, super-soldier Captain America destroyed and detonated the missile while airborne. The conflict on Mars prevented the Chitauri to sent a much larger invasion fleet on Earth. The Mars conflict saw the near-extinction of the Martian race, but leading to the Chitauri to retreat from the Solar System after its fleet were all nearly destroyed by the Martians' anti-orbital defense system.
By the early 21st Century, the Chitauri under Herr Kleiser that were repelled from Earth returned to invade Earth. But their invasion was again thwarted by the Avengers. Later, the Chitauri Federation were contacted by Ronan the Accuser, who informed that Earth remained unprotected and ample for a renew invasion from salvaging the Martian anti-orbital defense system - but unknown to the Chitauri, Ronan did not told them that Earth was under the protection of the Kryptonian hero Superman as part of his plan to be rid of his enemy. The Chitauri were yet again thwarted by Superman, the Justice League, X-Men, and Fantastic Four, and once again forced to r etreat from the Solar System.