Loki's Assault on New York
After the Cask of Ancient Winters was discovered on Earth, Loki passed this information first to the Dark Elves and then to the Frost Giants, promising each his help in attacking Earth and claiming the Cask for themselves. Loki appeared on Earth at the Statue of Liberty, causing SHIELD to send a response team. Loki allowed himself to be defeated but took mental control of several agents in the process. With their help he was able to steal the Phantom Zone Projector, the technology used by the Fantastic Four and Superman to lock away Zod and Annihilus in Nullspace. Combining the device with his magic, Loki ripped open portals to Jotunheim and Svartalfheim across New York letting the armies of Frost Giants and Dark Elves spill out into the streets. With the Fantastic Four off-world, Superman dead and the JLA scattered, Captain America, Iron Man, The Wasp, Ant Man, Thor and the Hulk work with SHIELD to contain the chaos and defeat Loki, keep the Cask out of the hands of both armies and use Reed Richards' device to send the armies back to their home dimensions and seal the rifts.
Dubbed the Avengers, the heroes formed their own team in NYC in order to protect Earth from future threats. They were soon joined by other heroes. In the aftermath of the battle Tony Stark held a press conference to unmask his secret identity and announce to the world that he was Iron Man.
During the same battle many street level heroes stepped up to defend their neighbourhoods in the midst of the chaos. Four of them, Daredevil, Jessica Jones, Iron Fist and Power Man, banded together in an informal alliance called (over protests of some of their members) the Defenders, in honor of the heroes of yesteryear. Other street level heroes joined and left the group's fluid roster.