Introduced in the pages of "The Ultimates" by writer Mark Millar and artist Bryan Hitch, the Chitauri are an alien race made up of shape shifters who slowly infiltrate a planet and, through the use of highly placed figures of power, manage to effect worldwide change. One of their earliest attempts at taking over the Earth was through their support of Adolf Hitler's Nazi regime. It was here that Captain America first fought against the Chitauri, as he helped end their plans to win the war for the Nazis through the creation of a nuclear bomb carried by an intercontinental ballistic missile. The missile was launched at the White House, but Captain America managed to climb on to the missile and destroy its guidance system, forcing it to crash harmlessly in the Arctic Ocean. Tragically for him, however, he crashed with the missile and ended up frozen in suspended animation.
Many decades later, Captain America was revived and instated as the center of a new team of superheroes known as the Ultimates. Financed by the international peacekeeping group S.H.I.E.L.D., the Ultimates consisted of a few heroes who gained powers mostly in attempts to recreate the serum that gave Captain America his abilities.
In the ensuing decades since World War II, the Chitauri had been slowly putting together a new scheme to take over the Earth. In order to achieve their objective, they infiltrated many key areas of S.H.I.E.L.D.. Nick Fury, the head of S.H.I.E.L.D., suspected this and had a secret group of operatives directly under his command weed out some of the S.H.I.E.L.D. spies.
This forced the Chitauri to accelerate their plans, luring a large group of S.H.I.E.L.D. agents and the Ultimates to a small Micronesian island. Once there, the Chitauri dropped a nuclear bomb on the island. Through the efforts of Iron Man and Thor, the Ultimates and a number of S.H.I.E.L.D. agents were saved, but many more S.H.I.E.L.D. agents were killed.
Thinking the Ultimates were dead, the Chitauri infiltrated the Ultimates' headquarters and took it over, taking the Wasp prisoner. The Ultimates then launched their counter-offensive, including a clever usage of the rather insane (but also insanely powerful) Hulk, by sending the rampaging monster after the Chitauri by implying that the Chitauri had questioned the Hulk's sexuality.
In the ensuing battle, Captain America encountered one of the Chitauri he had fought against during World War II. This villain, known as Herr Kleiser, was a Chitauri leader. Cap decided to fight him head on, even though the alien was significantly more powerful than he was. When it seemed that Cap was defeated, Kleiser taunted him by asking him for his surrender, leading to Captain America reacting with anger and one of the more famous lines of the series - the politically incorrect, "Surrender? SURRENDER? You think this letter on my head stands for France!"
At this point, the Hulk shows up and finishes Kleiser off by eating him. The Ultimates have saved the Earth from aliens and cemented themselves as worldwide heroes.