Sentinels are mutant-hunting robots. The typical Sentinel is three stories tall, capable of flight, projects energy blasts, and can detect mutants.
The Sentinel was conceived by S.H.I.E.L.D. that was initially developed as an anti-mutant countermeasure under the Sentinel Project. At the time, the rationale was that, sooner or later, mutants were to eventually be revealed to the public. If such an event were to happen, some mutants would proved themselves as a threat that conventional law enforcement would not be able to stop them. As such, the Sentinels were used as a method of law enforcement and served as the last resort in the worst-case scenario. S.H.I.E.L.D. approached Lionel Luthor to have his company to develop the weaponry.
Half-way through the Project, however, it didn't went through the production phase and was discontinued by General Nick Fury, who seen it as unethical and realized that equipping the Sentinels with deadly weapons and potentially falling into the wrong hands would turned them into engines of genocide. Despite its discontinuity, the project was secretly resurrected by military scientist Bolivar Trask, a anti-mutant sympathizer, and constructed approximately more than a hundred Sentinels for the Friends of Humanity. Its funding was secretly provided by a series of dummy corporations with ties to Shaw International and Van Damme Industries. Their CEOs, Sebastian Shaw and Doctor Doom, manipulated the project for their own ends to reveal mutants to the public.
Wolverine and Sabretooth were captured by the F.O.H. and were used as test subjects for several early Sentinel prototypes. These prototypes were also used against the X-Men and Superman, in which the battle exposed mutants to the public, but were destroyed. Graydon Creed, leader of the Friends of Humanity, planned to launch his Sentinel arsenal on the mutant island haven Genosha. However, the X-Men, Avengers, Fantastic Four and many other heroes destroyed half of the available Sentinels, which forced Creed to launch the remaining Sentinels on Genosha. Superman, Hawkgirl and Green Lantern John Stewart followed the Sentinels and managed to destroyed them before reaching Genosha.
The Sentinels that were destroyed by Superman and his allies were only then assimilated by Brainiac Mark VI and formed into a 200-foot tall Sentinel/Kryptonian machine hybrid that is heavily resilient to conventional attacks due to it being modified by Brainiac's technology. The Brainiac-Sentinel monstrosity acted out in hacking every computer and military systems on Earth and proceeded to New York state to find an efficient power source in order to launch all of the world's nuclear missiles to extinguish all life on the planet. The machine was finally disabled when Brainiac VI's brethren Mark V transmitted a computer virus which canceled its defenses and be destroyed by Superman, Batman, Green Lantern, Hawkgirl, Wonder Woman, Aquaman, Flash, and the Martian Manhunter.
The F.O.H. still possess some Sentinels in their arsenal and were later use in the occupation of Genosha.
- SX-01 - Designed to battle Superman; its main weapon is powered by a Kryptonite meteor fragment. It was ultimately destroyed by Superman after its Kryptonite-based weapon was made useless on his Kryptonite-proof nano-suit. Its Kryptonite was thrown out of Earth's orbit by Superman and directly towards the sun.