H.A.M.M.E.R. is a fictional peace-keeping agency featured in comic books published by Marvel Comics. They are part of the mainstream Marvel Universe and were introduced in the final issue of the Secret Invasion comic book limited series in January, 2009. H.A.M.M.E.R. served as a substitute for S.H.I.E.L.D., after it was decided that S.H.I.E.L.D. was no longer a reliable entity.
The ramifications of the alien Skrulls' secret invasion of Earth forced a massive restructuring of the United States' defense network. At the time, the country's primary peace keeping agency was S.H.I.E.L.D., led by civilian industrialist Tony Stark, aka, Iron Man. Because the Skrulls were able to compromise S.H.I.E.L.D.'s StarkTech technology, it was decided that S.H.I.E.L.D. was no longer an effective organization and Tony Stark was personally held accountable for not preparing adequately for the Skrull invasion. In the wake of this controversy, Thunderbolts leader Norman Osborn manipulated the U.S. government into allowing him to serve as director of a replacement agency called H.A.M.M.E.R. Under Osborn's leadership, H.A.M.M.E.R. had administrative control over the entire 50-State Initiative.
As the director of H.A.M.M.E.R., one of Osborn's first initiatives was to issue warrants, demanding the arrest of former S.H.I.E.L.D. director Tony Stark and deputy director Maria Hill. Moreover, Osborn wanted access to the database containing the identities of every hero registered under the Superhuman Registration Act. To prevent Osborn from getting his hands on such vital information, Tony Stark took measures to erase all known resources containing this sensitive information, including his own mind.
When Stark failed to surrender himself to Osborn, Norman sent contingents of H.A.M.M.E.R. squads to Stark Industries facilities all across the globe with orders to arrest Tony Stark with extreme prejudice. The militant agents caused substantial damage in their efforts to find Stark, often brutalizing Stark Industries' personnel.