Chitauri Invasion

"The Chitauri grow restless." "Let them grow restless. I will lead them in the 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 Invasion was one of the first wars between humanity and beings from other worlds. The war lasted for less than a week, but the impact it left was great. Taking place on Earth, the war was fought by the exiled Asgardian prince Loki and his Chitauri allies against the applicants of the Avengers Initiative and the espionage agency known as S.H.I.E.L.D. At first the members of the team refused to cooperate with each other, but after Phil Coulson's demise at the hands of Loki, they were able to work together and defeated the Chitauri at the Battle of New York.

Prelude
The events of the war were set in motion shortly after the Battle of Puente Antiguo, when a fight between Thor and Loki in Asgard resulted in the destruction of the Bifrost Bridge. This caused a temporal anomaly that swallowed up Loki, teleporting him to a region of space virtually unknown to the Asgardians. Here, he met Thanos. Through interrogation, Thanos learned of the history regarding Loki's people, but the mention of the Tesseract was what truly piqued his interest. The Tesseract was, in fact, one of the Infinity Stones, a collection of powerful objects that Thanos had been seeking; why Thanos desired them is as of now unclear. Seeing a chance for greatness, Loki brokered a deal with Thanos, offering to give him the Tesseract, which was being studied by S.H.I.E.L.D. on Earth, in exchange for an army to conquer Earth. Initially skeptical, Thanos agreed and granted Loki leadership over the Chitauri, a powerful, technologically advanced alien race.

Over the course of the following year, Loki planned out his invasion, controlling scientist Erik Selvig from afar and using him as a vessel to study Earth and the threat its inhabitants may pose. In the year 2012, Loki was finally ready to begin his invasion, and was given a Scepter by Thanos. The scepter contained another Infinity Stone that had the ability to control the minds of others, and would aid Loki greatly in his mission. Then, through manipulation of cosmic energy, Thanos teleported Loki to Earth.

The Beginning of the War
"Coulson, get back to base. This is a Level 7. As of right now, we are at war."

- Nick Fury

At Project P.E.G.A.S.U.S., a remote research facility where the Tesseract was being held, the Tesseract began spiking large quantities of energy, forcing S.H.I.E.L.D. to evacuate the facility. Nick Fury, the director of S.H.I.E.L.D., arrived at the facility with his second-in-command, Maria Hill. Phil Coulson explained the situation to them, but before anything could be done, the Tesseract opened a portal from which Loki emerged. Using the scepter, Loki took control of Selvig and Hawkeye, who aided him in stealing the Tesseract and escaping the facility. Shortly thereafter, the facility was obliterated by unstable energy left behind from the portal.

Assembling the Avengers
"This is out of line, Director. You're dealing with forces you can't control." "You ever been in a war, Councilman? In a firefight? Did you feel an overabundance of control?" "You saying that this Asgard has declared war on our planet?" "Not Asgard. Loki."

- The World Security Council and Nick Fury

In response to the attack, Fury reactivated the Avengers Initiative, which was shut down a year prior due to lack of global crisis. Agent Natasha Romanoff recruited Dr. Bruce Banner in India, while Agent Coulson visited Tony Stark and requested that he review Selvig's research. Fury himself approached Steve Rogers with an assignment to retrieve the Tesseract from Loki; Rogers was familiar with the Tesseract as it played a vital role in his World War II exploits. Romanoff, Coulson, Rogers, and Banner all met the following day on the Helicarrier moments before it went airborne. Dr. Banner was escorted to his lab after a brief meeting with Fury; Banner had been consulted to trace the gamma signature that the Tesseract radiated. After spending some time tracking the whereabouts of the Tesseract on the Helicarrier, a man of Loki's description was located in Stuttgart, Germany.

Capturing Loki
"Loki is beyond reason, but he is of Asgard and he is my brother." "He killed eighty people in two days." "He's adopted."

- Thor and Black Widow

Romanoff and Rogers, as the Black Widow and Captain America respectively, traveled to Stuttgart to apprehend Loki, who, with the aid of the mind-controlled Hawkeye, was stealing iridium needed to stabilize the Tesseract and demanding worship from a crowd of terrorized civilians. Loki and Rogers battled briefly, but their bout was interrupted by Iron Man, whose appearance prompted Loki's surrender. While the group was transporting Loki to the Helicarrier, Thor suddenly appeared, having been transported to Earth by Odin, and attempted to free Loki and convince him to return to Asgard. After a confrontation with Iron Man and Captain America, Thor agreed to work with the humans and accompanied them to imprison Loki on the Helicarrier.

After Loki was put into the cell originally intended for the Hulk, Thor revealed to the team that Loki planned on using the Tesseract to open a portal that would unleash the Chitauri upon Earth, commencing a planetary takeover. Suspicious of S.H.I.E.L.D.'s true intentions regarding the Tesseract, Stark had his AI assistant, J.A.R.V.I.S., hack into the agency's database. Rogers was less than thrilled at Stark's behavior, but found himself equally suspicious and investigated himself. The two eventually uncovered Phase 2, a top secret program dedicated to building weapons powered by the Tesseract's energy. While all of this is going on, Romanoff spoke one-on-one with Loki, and eventually discovering that he planned on using the Hulk as a means of escaping the Helicarrier.

Loki's Escape
"You threaten my world with war. You steal a force you can't hope to control. You talk about peace and you kill cause it's fun. You have made me very desperate. You might not be glad that you did."

- Nick Fury to Loki

After all of these things came to light, the Avengers became divided over how to deal with Loki and the revelation that S.H.I.E.L.D. planned to harness the Tesseract’s power to develop weapons of mass destruction. Fury admits that the Battle of Puente Antiguo 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 argued, Hawkeye and Loki’s possessed agents attacked the Helicarrier, disabling its engines. As Iron Man and Captain America attempted to restart the damaged engines, Banner transformed into the Hulk, despite Romanoff's efforts to calm him, and ran amok inside the ship, soon battling Thor.

After the battle, the Hulk was thrown from the Helicarrier and crashed into a warehouse. During a fight with Hawkeye, Romanoff freed her comrade from Loki's control by delivering a concussive strike to his head. Loki, meanwhile, escaped his cell, trapping Thor inside when he attempted to stop him. Coulson confronted Loki in an attempt to rescue Thor, but Loki stabbed Coulson with the scepter and left him for dead. After ejecting Thor from the Helicarrier, Loki departed from the ship. Fury attempted to save the expiring Coulson, but was to late as the agent succumbed to his injury and died.

The Final Battle
"Stark, you seeing this?" "Seeing, still working on believing."

- Captain America and Iron Man

After mourning Coulson's death, Stark and Rogers worked out that Loki would orchestrate the alien invasion at Stark's New York-based home, Stark Tower, as simply defeating them would not be enough for Loki; he needed to overpower them in a public way so as to validate himself as ruler of Earth. Using a Tesseract-powered interdenominational generator Selvig had built, Loki opened a portal to the Chitauri fleet over Manhattan, summoning the invasion. The Avengers rallied in defense of New York, but quickly concluded they would be overwhelmed as wave after wave of Chitauri and Leviathans descended. The Avengers kept their ground to hold off the invasion and evacuated civilians while the Hulk pursued Loki and beat him into submission. Romanoff made her way to Selvig's device, where Selvig, freed of Loki's control, revealed that Loki's scepter could be used to close the portal.

Meanwhile, Fury's superiors attempted to end the invasion by launching a nuclear missile at Manhattan. Despite Agent Hill and Fury's orders to not fire, a rogue jet launched the missile at the city. Iron Man intercepted it and took it through the portal toward the Chitauri Mothership, before running out of power and plummeting back to Earth. He returns to the Earth's atmosphere just as the portal closes. The Chitauri are decimated by the missile destroying the Mothership. The Avengers then apprehended Loki, thus ending the war.

Aftermath
"Everything's changing. A little while ago, most people went to bed thinking that the craziest thing in the world was a billionaire in a flying metal suit. Then aliens invaded New York and were beaten back by among others, a giant green monster, a costumed hero from the 40's, and a god."

- Maria Hill

While the war had ended in humanity's favor, the consequences were numerous. News channels following the war confirmed the legitimacy of the extraterrestrial attack and some Americans regarded the Avengers with praise and thanks while others called for their arrests, finding the convenience of their appearance to be suspicious. The conflict also left an emotional toll on various figures, such as Tony Stark and Erik Selvig, the former whose experiences during the battle left him with post traumatic stress disorder, and the latter was left mentally scarred after his time under Loki's control. Regarding Loki, Thor escorted him and the Tesseract back to Asgard to take responsibility for his crimes. He would eventually be incarcerated in Asgard's dungeons by Odin, but would eventually be released by Thor to assist in defeating the Dark Elves. In that conflict, Loki faked his own death so that he could usurp the Asgardian throne from Odin without his brother's knowledge. The scepter that Loki used during the conflict was believed to have been confiscated by S.H.I.E.L.D.; yet, through HYDRA's moles in S.H.I.E.L.D., it was obtained by Wolfgang von Strucker, whose scientists used it to activate the powers of Pietro and Wanda Maximoff.

The extensive repairs cost New York $160 billion, and various pieces of Chitauri technology and paraphernalia were picked up by ordinary people and found their way into the black market. These issues, added with the revelation that humanity was not alone in the universe, caused S.H.I.E.L.D. to organize a special team that would deal with other supernatural phenomena. Agent Coulson, who was revived after his death at the hands of Loki, became the leader of the team. The greatest threat faced by his team, a shady organization known as the Centipede Project, a scientific offshoot of HYDRA, used samples of Chitauri tech to create the Centipede formula, which was used to create super soldiers. After various conflicts, the project was brought down by S.H.I.E.L.D. with the demise of their leader, John Garrett, but some of its members managed to escape.

The Avengers went their separate ways after the battle, and Fury did them the courtesy of not tracking their whereabouts, feeling that they had contributed much to the human race during the war. He remained confident that they would return if a new world-threatening menace emerged.

Ironically, the next world-threatening force that the Avengers would face stemmed directly from the ruins of the Battle of New York: a computer program that would eventually become Ultron, an advanced artificial intelligence perfected by Tony Stark to be the ultimate peacekeeping force of the human race. Ultron would eventually go rogue and cause even greater harm to the planet than Loki did.

Meanwhile, Thanos, while disappointed with Loki's failure, resumed his hunt for the other Infinity Stones, sending his emissaries out across the galaxy to find them. His search brought him into conflict with another team of individuals, the Guardians of the Galaxy.