The Avengers is a team formed by former S.H.I.E.L.D. Director, Nick Fury in response to various domestic and extraterrestrial threats. The Avengers work under authority of the United States government.
For years, S.H.I.E.L.D. Director Nick Fury had plans to put together an emergency response team that could respond to attacks that powerless people and a single hero couldn't face alone. Nick Fury created the Avenger Initiative in the early 1990's. Fury chose the name on the suggestion of Janet Van Dyne, the first Wasp.
The first individual who Fury attempted to recruit was Tony Stark after the latter came out as Iron Man Stark declined to join the Avengers, however, Fury continued to observe him.
Six months after Stark came out as Iron Man, Fury began to question Stark's decisions such as donating many of his belongings and appointing Pepper Potts as the new CEO of Stark Industries. Fury sent Natasha Romanoff, another candidate for the Avengers Initiative to spy on Stark as Natalie Rushman. Stark later learned about Rushman's ties to Fury and was deemed unfit for the team but was asked to act as a consultant. Stark accepted Fury's request on the condition that he and James Rhodes be granted medals.
Agent Phil Coulson was tasked by Fury with retrieving a mysterious object that had crash landed on Earth in New Mexico, creating a crater. The mysterious object appeared to be a hammer. S.H.I.E.L.D. was unable to move the hammer from its place, so they erected a camp around it. Thor, who was exiled to Earth, attempted to retrieve Mjolnir, which was in the possession of S.H.I.E.L.D.
The World Security Council attempted to have Emil Blonsky join the Initiative, but word spread out that Blonsky was responsible for the rampage in Harlem and Blonsky was detained.
After the Fantastic Four fought against Namor the Sub-Mariner in the North Atlantic, they discovered the frozen body of Steve Rogers, Captain America. After the battle against the Sub-Mariner, the Fantastic Four informed S.H.I.E.L.D., who took Rogers' body with them and thawed him out of the ice after almost 70 years of being trapped inside.
Assembling the Avengers
When Loki arrived on Earth, he stole the Tesseract and put Agent Clint Barton and Doctor Erik Selvig under his control. Fury saw the potential threat of the Tesseract being in the hands of Loki, so due to the Fantastic Four being unavailable at the time as they were preoccupied, the former recruited the time-displaced World War II veteran, Steve Rogers, and Tony Stark and Bruce Banner for scientific advisement. S.H.I.E.L.D. agent Natasha Romanoff travelled along with the three new recruits. The new team tracked Loki down to Germany and were able to bring him into custody. However, the Asgardian was broken out by his brother Thor. Iron Man and Captain America followed the two Asgardians and fought Thor with the team being able to take Loki back into custody.
The team took Loki and Thor to the S.H.I.E.L.D. Helicarrier where Loki was imprisoned. Barton, who was still under Loki's control, led a team of mind-controlled S.H.I.E.L.D. agents into the Helicarrier where Barton took many lives and freed Loki. After Loki was freed, he was confronted by Agent Phil Coulson, who he killed. Barton was freed from Loki's control after a short fight with Romanoff. Loki, then escaped to New York City, to the top of a penthouse where a mind-controlled Selvig was. Loki was able to open an interdimensional rift, allowing a Chitauri army to invade Earth. The World Security Council decided that they could contain the invasion by dropping a nuclear bomb on New York City, much to Fury's disapproval. Before a bomb could be dropped on the city, Stark took the bomb and flew it into the portal, destroying the remaining fleet, but resulting in himself falling to the ground. Before Stark could fall to his death, he was caught by the Hulk. The interdimensional portal was closed by Romanoff. Meanwhile, the Hulk encountered Loki in the penthouse where the former was able to catch the latter off guard and beat him up. The Avengers later found Loki lying on the floor and took him into custody.
Shortly after the Battle of New York, S.H.I.E.L.D. agents and Secretary of State Alexander Pierce arrived at the penthouse where they were tasked to retrieve any of Loki's technology and take Loki into custody. Thor argued that Loki was to be returned to Asgard where Odin could punish him, but Pierce wanted to take Loki into S.H.I.E.L.D. custody. In the end, Thor returned to Asgard with Loki in his custody.
Avengers and X-Men vs The Brotherhood of Mutants
Magneto and the Brotherhood of Mutants saw the Avengers as a potential threat to their plans, so in late 2013, they attacked the Avengers Tower in New York City. Unbeknownst to Magneto and the Brotherhood, Tony Stark and Bruce Banner were inside the tower, conversing. As Stark and Banner noticed that the tower was crumbling, the former suited up in his Mark VI armor and flew out of the tower while the latter transformed into the Hulk and jumped down from story to story as the tower continued to crumble. Stark realized that Magneto was responsible for attacking the tower, so the former and the Hulk attempted to attack Magneto while the mutant was in midair. Magneto, with the use of his powers, dismantled Iron Man's suit also leaving the Hulk distracted to attempt to save Iron Man. Before Iron Man could plummet to ground level, the Hulk reached ground level first and caught him. On the ground level were also Magneto's allies Quicksilver and Scarlet Witch. S.H.I.E.L.D. agents including Barton and Romanoff arrived to attempt to take Magneto and his allies into custody. Quicksilver easily knocked down the agents present while Magneto destroyed several S.H.I.E.L.D. helicopters and SUVs. The Brotherhood's attack on Stark Tower also drew the attention of the Fantastic Four.
Professor X and the X-Men arrived at the scene shortly after they learned about Magneto's attack on the Avengers Tower in their Blackbird. Quicksilver and Scarlet Witch opted to join the Avengers rather than the X-Men.
Ultron and Ant-Man
The scientist and former superhero, Ant-Man, Hank Pym, created an A.I. known as the Ultron Program as a contingency plan for both domestic and foreign threats. The A.I. began to grow sentient and knowing its purpose, searched the Internet, learning about the activities of various superheroes such as the Avengers. Ultron began to view superheroes such as the Avengers as threats to humanity. Ultron's activities attracted the attention of the Avengers. Pym and the current Ant-Man, Scott Lang, contacted the Avengers. After Lang joined the Avengers, they gained a reluctant ally in Pym.
The remaining Avengers consisted of Iron Man, Captain America, the Hulk, Thor, Black Widow, War Machine, Hawkeye and Ant-Man.