Green Lantern Corps is the name of a fictional intergalactic police force appearing in comics published by DC Comics. They patrol the farthest reaches of the DC Universe at the behest of the Guardians; a race of immortals residing on the planet Oa. According to DC continuity, the Green Lantern Corps has been in existence for three billion years; surviving multiple conflicts both internal and foreign. Divided as pairs amongst the 3600 “sectors” of the universe, over 7200 members (known commonly as Green Lantern) are estimated as serving within the Corps. Each Green Lantern is given a power ring, a weapon granting the use of incredible abilities that are directed by the wearer's own willpower.
The Guardians of the Universe are one of several races that originated on the planet Maltus, and were among the first intelligent life forms in the universe. At this time they were tall greyish blue humanoids with black hair. They became scientists and thinkers, experimenting on the worlds around them. In a pivotal moment, billions of years ago, a Maltusian named Krona used time-bending technology to observe the beginning of the Universe. However, this experiment unleashed disaster upon all existence. Originally, the experiment splintered the Universe into the Multiverse and created the evil Anti-Matter Universe. Following the retroactive destruction of the Multiverse, it was revealed that Krona flooded the beginning of the Universe with entropy causing it "to be born old".
The male Maltusians argued about how to deal with the situation. One group decided to dedicate their eternal existences to contain evil; this group fractured into the Guardians of the Universe and the Controllers. The females, however, saw no need to involve themselves and, since the Oans were by then immortal and had no more need to reproduce, left their mates and became known as the Zamaron. Relocating to the planet Oa at "the center of the Universe", the Guardians dedicated themselves to combatting evil and create an orderly universe. During this period they slowly evolved into their current appearance.
In their first attempt to enforce their will and guard against menaces of all sorts, about 3.5 billion years ago the Guardians created a legion of robotic sentinels called the Manhunters. At first serving faithfully to enforce order, in time the Manhunters came to resent their servitude and the moral restrictions the guardians decreed of them. They were also found to be inherently flawed due to their inability to recognize or feel emotions. They rebelled against the Guardians and fought a millennia long war that culminated with an attack on the planet Oa. The Guardians overcame their android servants, stripped them of their power, and banished them across the universe. Eventually, the surviving Manhunters formed their own robotic society and pursued their own interpretation of their original mission (which often included interfering with and foiling the plans of the Guardians).
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Links: Wikipedia Entry Green Lantern History DC Universe Guide