S.W.O.R.D. (Sentient World Observation and Response Department), which works with S.H.I.E.L.D. but specializes in extraterrestrial threats. Dialogue in the stories depicting both organizations has been ambiguous on whether S.W.O.R.D. is a branch of S.H.I.E.L.D. or a sister agency.
S.W.O.R.D. is most commonly managed by Abigail Brand, daughter of an extraterrestrial father and a mutant mother. Her mutant ability to ignite her hands in a blue flame hot enough to melt most metals, and the alien physiology of her tongue allows her to speak alien dialects humans can't.
S.W.O.R.D.'s base of operations is a orbital space station dubbed The Peak. It has gone through many iterations as it is often the first thing attacked by alien invaders.
Breakworld was a technologically advanced planet capable of seeing versions of the future. They believed Colossus was destined to destroy their planet and decided to strike first. SWORD controversially agreed to let them try to "cure" the mutant gene but also secretly worked with the X-Men to stop their doomsday device.
During the invasion of Skrulls, The Peak was destroyed by a Skrull posing as Dum Dum Dugan. Luckily, Brand was able to board a Skrull ship and released Reed Richards.
Post Secret Invasion
After Secret Invasion, Spider-Woman was recruited as a field agent after infamously being kidnapped and replaced by Skrulls. Her first mission is to capture Skrulls hiding in Madripoor which brings her into direct conflict with both Hydra and the Thunderbolts.
Also after Secret Invasion, Henry Peter Gyrich was assigned to SWORD to assist in mass deportations of Earthbound aliens, regardless if they are peaceful or traveled here legally, based on a law Brand disagreed with.
SWORD would reach out to the Avengers Unity Squad when they came into possession of video recording of the Apocalypse Twins killing a Celestial before making their way to Earth. The twins targeted peak to create a diversion while they attacked the Akkaba Metropolis.
Alpha Flight would eventually replace SWORD as Earth's orbital defense agency, and Brand would transfer to it until the mutants brought back SWORD.
Mutant Space Program
When Xavier and Magneto tamed Krakoa into a safe nation for mutants, they quickly expanded their ability to defend themselves. One such expansion was taking over the defunct space station of SWORD, The Peak. With long time director and half-alien mutant, Abigail Brand, they put together a new system of security, observation, and diplomacy with off-world civilizations centered around their unique mode of teleportation, six mutants (The Six) using their abilities in tandem for more significant results, similar to the way The Five work.
In other media
S.W.O.R.D. appears as a government agency in the MCU streaming series WandaVision. It has been renamed Sentient Weapon Observation Response Division, and it has closed down its space activity to turn its sights on artificial intelligence. It was founded by Maria Rambeau.