For some time, S.H.I.E.L.D. had been documenting and cataloging super-humans of interest or who posed a threat to the global government. Thus, the organization founded the Avengers, but over time, Earth's most powerful heroes were swept up in a heroic fervor that inspired them to act as public figures across the globe, which made them significantly more difficult to control.
Still, the organization needed a superhuman task force to carry out missions that the Avengers would not be satisfied to participate in. Gradually, names of vigilantes and potential vigilantes who were willing to serve their country and world governments, true agents of the state, were gathered.
With the help of Dr. Hank Pym, who agreed to leave the main Avengers team to serve as chief of operations for this new group, the team was quickly assembled. Hawkeye and Black Widow were enemies of the state who agreed to work alongside the organization in exchange for their freedom. US Agent and Mockingbird were official agents of the US government, so they didn't hesitate when they were called to join the group. Tigra and War Machine were vigilantes who acted on their own, but realized that alongside S.H.I.E.L.D. they would be able to do much more. And Quake and Wonder Man were young recruits who had been taken in by the organization with the help of the Avengers, and who were inspired to pursue heroic careers.