The Marvel Book Expand Your Knowledge of a Vast Comics Universe by Stephen Wiacek

Defend us from evil
However, the most persistent, varied, and arguably effective grouping is not
actually a team at all. Calling themselves the Defenders, these champions
from society’s fringes reluctantly convene when the planet or universe are
menaced. The ad hoc alliance first forms after Doctor Strange falls battling
the imminent invasion of the primordial Undying Ones. He is (almost
accidentally) rescued when the demons subsequently target Namor the Sub-
Mariner, and the Hulk. Some months later, Namor asks the Hulk and Silver
Surfer to help him stop an American weather experiment that Atlantean
scientists predict will end in atomic Armageddon. The proud prince only
seeks allies because Earth is endangered and the project is guarded by the
patriotic, blithely unaware Avengers. Despite disliking each other, the
antiheroes often reunite when a situation demands: eventually working with
almost every hero (and a few villains). These Defenders regard themselves
as a “non-team,” even though frequent collaborators Nighthawk and
displaced Asgardian warrior Valkyrie continually agitate to formalize the
arrangement. After the “founders” eventually go their separate ways, one
iteration eventually achieves that aim, but the X-Men-heavy New
Defenders ultimately split up, too.
Doctor Strange later orchestrates a fluctuating roster of magically selected
Secret Defenders to avert specific disasters and, over the years, repeatedly
reunites with Namor, Hulk, and the Surfer as universal perils warrant. Other
teams Strange works with also co-opt the Defenders’ name for brief periods
and their own team projects. For a time, it is the appellation used by Luke
Cage, Jessica Jones, Iron Fist, and Daredevil as they seek to clean up the
streets of New York.
Whether wearing a uniform and following a formal code of conduct, or just
hanging out as a gang of like-minded loners, forming a united force to face
threats to Earth is the ultimate Super Hero goal.