Chitauri Invasion

The War for Earth was one of the first wars between Earth and other planets. The war lasted for four days. It started when Loki attacked Project P.E.G.A.S.U.S. and controlled the minds of Hawkeye and Erik Selvig, which lasted for most of the war. Nick Fury felt that this was the perfect time to initiate the Avengers Initiative. At first the members of the team didn't get along, but after Phil Coulson's fake death they learned how to work and fight together in the Battle of New York, which ended the war.

Beginning of the War
Nick Fury, the director of the espionage agency known as S.H.I.E.L.D. arrives at Project P.E.G.A.S.U.S., a remote research facility, during an evacuation. His second-in-command, Maria Hill, explains that the Tesseract, an energy source of unknown potential, has activated and opened a portal through space, from which the exiled Asgardian prince Loki, steps through. Loki takes the Tesseract, and uses a scepter to control the minds of several S.H.I.E.L.D. personnel including Agent Clint Barton, and physicist consultant Dr. Erik Selvig in order to aid in his getaway.

Bringing the Avengers Together
In response, Fury reactivates the Avengers Initiative. Agent Natasha Romanoff recruits Dr. Bruce Banner in India, while Agent Phil Coulson visits Tony Stark and requests that he reviews Selvig's research. Fury himself approaches Steve Rogers with an assignment to retrieve the Tesseract from Loki; Rogers is familiar with the Tesseract as it played a vital role in his World War II exploits.

Romanoff, Coulson, Rogers, and Banner meet on the Helicarrier before it goes airborne. Dr. Banner is escorted to his lab after a brief meeting with Fury; Banner has been consulted to trace the gamma signature that the Tesseract radiates. After spending some time tracking the whereabouts of the Tesseract on the Helicarrier, a man of Loki's description is located in Europe.

Capturing Loki
Iron Man, Captain America, and Romanoff travel to Stuttgart, Germany to apprehend Loki, who, with the aid of a mind-controlled Barton, is stealing iridium needed to stabilize the Tesseract and demanding worship. After a battle with the heroes, Loki surrenders and is being returned to the Helicarrier in the Quinjet. Thor, Loki's adoptive brother and the Asgardian god of thunder, attempts to free Loki and reason with him. After a confrontation with Iron Man and Captain America, Thor accompanies them to imprison Loki on the Helicarrier.

After Loki is put into the cell originally intended for the Hulk, Thor reveals to the team that Loki is allied with the Chitauri, a powerful and technologically-advanced extraterrestrial race unknown to both Earth and Asgard. Stark has J.A.R.V.I.S. hack into the S.H.I.E.L.D. database while he works with Dr. Banner. Rogers is less than thrilled at Stark's behavior and tells the two scientists to resume their work and investigates Phase Two, S.H.I.E.L.D.'s top secret weapons program. While all of this is going on, Romanoff speaks one-on-one with Loki, and eventually discovers that he plans on using the Hulk as a means of dismembering the team.

Loki's Team Attacks the Helicarrier
After all of these things come to light, the Avengers are divided over how to deal with Loki and the revelation that S.H.I.E.L.D. plan to harness the Tesseract’s power to develop weapons. Fury admits that the events in New Mexico a year before made S.H.I.E.L.D. aware of other extraterrestrial races that may threaten Earth. Weapons developed from the Tesseract would form a means of deterrence. As the group argues, Barton and Loki’s possessed agents attack the Helicarrier, disabling its engines. As Iron Man and Captain America attempt to restart the damaged engines, Banner transforms into the Hulk, despite Romanoff's efforts to calm him, and runs amok inside the ship, soon battling Thor. During a fight with Barton, Romanoff discovers that knocking him unconscious breaks Loki's mind control. Loki escapes his cell with the help of a possessed agent and traps Thor in the cell. Coulson confronts Loki in an attempt to rescue Thor, and Loki then stabs Coulson through his back with his staff before ejecting Thor from the ship. Fury confronts the expiring Coulson, who hopes his death can be used to motivate the Avengers into working as a team.

Ending the War
Stark and Rogers work out that Loki will orchestrate the alien invasion at Stark's New York-based home, Stark Tower, and that defeating them will not be enough for Loki; he needs to overpower them in a public way so as to validate himself as ruler of Earth. Using a Tesseract-powered interdenominational generator Selvig has built, Loki opens a portal to the Chitauri fleet over Manhattan, summoning an invasion. The Avengers rally in defense of New York, but quickly conclude they will be overwhelmed as wave after wave of Chitauri and Leviathans descend. The Avengers keep their ground to hold off the invasion and evacuate civilians while the Hulk beats Loki into submission. Romanoff makes her way to Selvig's device, where Selvig, freed of Loki's control, reveals that Loki's staff can be used to close the portal.

Meanwhile, Fury's superiors attempt to end the invasion by launching a nuclear missile at Manhattan. Despite Agent Hill and Fury's orders to not fire, a rogue jet launches the missile at Manhattan. Iron Man intercepts it and takes it through the portal toward the Chitauri fleet before running out of power and plummeting back to Earth. The Hulk catches him as he falls. The Avengers then apprehend Loki, who is still weak from his encounter with the Hulk. News channels confirm the legitimacy of the extraterrestrial attack and some Americans regard the Avengers with praise and thanks while others call for their arrests. Thor escorts Loki and the Tesseract back to Asgard to take responsibility for his crimes. Fury notes that the Avengers will go their separate ways until such time as a new world-threatening menace emerges.