Assembled by Captain America during Civil War, his group, dubbed the "Secret Avengers", was an underground organization of heroes resisting the Super-Human Registration Act.
Mark Millar, writer of Civil War, first created the Secret Avengers in the pages of the main event.
Major Story Arcs
For further details: Civil War
When Captain America created an underground group in opposition to the Superhuman Registration Act, this group was unofficially dubbed his Secret Avengers. This resistance was aided by the resources Nick Fury and Cable, and they operated out of a S.H.I.E.L.D. safehouse hidden in Manhattan. Those who initially flocked to Captain America's side were mostly street-level vigilantes such as Cloak, Dagger, Luke Cage, Falcon and Danny Rand posing as Daredevil. But other more powerful heroes like former allies Hercules and Goliath chose to stand with Captain America as well.
At first, the team's only intent was to continue fighting crime while evading capture by law enforcement. They soon escalated their activities by ambushing law enforcement personnel attempting to detain the Young Avengers. The teen heroes were then folded into the Secret Avengers' ranks. The group was later lured into a confrontation with Iron Man's pro-registration forces, and in the ensuing violence, Goliath was killed by a Thor replicant later known as Ragnarok. This tragedy bolstered support and membership of Captain America's resistance as several former Avengers and New Warriors joined up. The Invisible Woman, Human Torch and Spider-Man also switched sides to become part of the resistance, and most controversial of all, the Secret Avengers accepted the Punisher into their ranks.
The Civil War culminated with the Secret Avengers' attack on the Prison 42 facility in the Negative Zone. With the help of Black Panther and by using Hulkling to infiltrate Iron Man's forces, the Secret Avengers were able to gain access to the prison and orchestrate a prison-wide breakout. This led to an immense battle with the pro-registration forces that quickly spilled out into the streets of Manhattan. The battle ended with Captain America surrendering after witnessing all of the collateral damage the fight had caused and deciding the resistance was not the way to oppose the SHRA.
In the aftermath, many of Captain America's Secret Avengers accepted the amnesty offered to them. Some did not, and several of those heroes went on to to be the New Avengers.
Coined as the Secret Avengers more by the media than any official declaration, the team was a large force of heroes that consisted of nearly every one who refused to register and had not already been arrested for it. These Secret Avengers lasted only as long as the Civil War event did, and what remained of this team was adopted by Brian Michael Bendis to be carried on in the New Avengers series.
Ed Brubaker revived the team name for use as Steve Rogers' covert unit of Avengers that he began using after becoming the new top cop of the Marvel Universe. This team's connection to the anti-registration group was in name only.