Leader
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Members
Aa, Alia, Amanita, Arisia, Ash, Boodikka, Brik, Cary Wren, Chaselon, Chriselon, Chthos-Chthas Chthatis, Eddore, G'nort, Green Man,
Guy Gardner,
Hal Jordan, Honnu, Iolande, Isamot Kol, Jack T. Chance,
John Stewart, Ke'Haan, Kilowog, Laira, Larvax, Markot Five, Medphyll, Mogo, R'amey Holl, Salakk, Sendrina, Soranik Natu, Stel, Tanakata Z, Tomar-Tu, Torquemada, Vath Sarn, Zax
Sinestro, Abin Sur, Adara, Ahtier, Arkkis Chummuck, Ch'p, Flodo Span, Galius Zed, Hollika Ran, Katma Tui, Kreon, Myrrt, Pathavim Seth-Ottarak, Tomar-Re, Yalan Gur

In the early days of the Universe, natives of the overpopulated planet Maltus evolved into immortals of great power. They subsequently settled the planet Oa and declared themselves the Guardians of the Universe and enemies of evil after one of their own, the renegade scientist Krona, performed a forbidden experiment that had terrible consequences for the Universe at large.
Exactly what those consequences were have varied with different versions of the stories; originally, it was supposed to have unleashed Evil in the first place. Later, it was attributed to have created the Antimatter Universe of Qward. Later still, it was used as the explanation for the existence of parallel universes in the DC Universe (and the creation of the Monitor), which eventually led to the rearrangement of Time itself, as seen in the Crisis on Infinite Earths. Finally, it has been stated that it accelerated entropy, shortening the duration of the universe by a billion years.
Some of the Guardians, however, disagreed on how to deal with the chaos that had been unleashed. One group, calling itself the Controllers, separated itself from the others and favored using more violent methods to achieve their ends. They would later found the Darkstars organization. The female Oans, feeling no need to involve themselves in the situation, also left, becoming later known as the Zamarons.
To enforce their will and guard against alien menaces of all sorts, the Guardians had created a legion of robotic sentinels called the Manhunters about 3.5 billion years ago. The Manhunters eventually came to resent their servitude and were also viewed as flawed due to their inability to recognize and/or feel fear. They rebelled against the Guardians and fought a millenia long war that culminated with an attack on the planet Oa. Ultimately, the Guardians overcame their android servants, stripping them of their power and banishing them across the universe. Eventually, they formed their own robotic society where they pursued their own agenda (which often included interfering with and foiling the plans of the Guardians).
Chastened by the failure of the Manhunters, the Guardians decided that their newest force of soldiers for good would consist of living beings, ones who had free will and strong moral character. To arm this new legion of celestial knights, the Guardians created the Power Rings, rings of inconceivably-advanced technology that allowed their wearers to project green beams of energy with which the bearer could conjure objects of any size or shape, limited only by his or her imagination and willpower.
Although the connection between the Guardian Oans and the origin of the Green Lantern symbol has yet to be clarified, it has been revealed that the lantern itself has its roots in the first life in the universe. Allegedly, the first life in the galaxy emerged on an unnamed foggy world, and when they developed a police force - the first in the universe - the constabulary carried a lantern lit with a green chemical flame, hence a corp of Green Lanterns. It is intimated that this ideal was adopted and spread throughout the galaxy as a symbol for law and justice and its ability to pierce confusion and uncertainty, eventually resulting in the formation of the Green Lantern Corps, with requisite power rings and lantern-shaped batteries along traditional lines. Initially consisting of only a few dozen agents at a time, the Corps radically increased its numbers approximately 1000 years ago. The Guardians sought to bring order to the planet known as Apokolips. Home to Darkseid, a galactic tyrant, Apokolips was and is a stronghold of evil. Green Lantern Raker Qarrigat sought to remove Darkseid from power, only to be humbled by Darkseid's might. Raker returned to Oa, where he convinced the Guardians that taking Apokolips would require an army. Authorized to start a massive recruitment, Raker and his fellow Lanterns inducted thousands of new members, bringing total membership up to 3600.
Both the Corps and the Guardians suffered casualties during the Crisis on Infinite Earths. The Guardians' failure to take appropriate action during the Crisis led to their decision to depart Oa in the company of their female counterparts, the Zamarons. Left to their own devices, the Corps underwent a major reorganization. A team of Green Lanterns led by Hal Jordan was stationed on Earth, and the system of assigning one Green Lantern to a sector was temporarily abandoned. The decision of the Corps to execute Sinestro resulted in the activation of a previously unknown fail-safe that depowered the rings of every Green Lantern except for Hal Jordan, Guy Gardner, G'nort and Ch'p.
The Guardians eventually returned to Oa and began the reconstruction of the Corps, assigning Guy Gardner to Earth, John Stewart to the Mosaic World, and Hal Jordan to recruit new members. Ironically, Jordan himself would eventually be responsible for destroying the incarnation of the Corps that he had helped create.
The Green Lantern Corps patrolled the DC Universe for over three billion years. In that vast length of time, some Green Lanterns rebelled and fell into evil. A rogue Green Lantern, Universo, was briefly shown to exist in the future epoch of the Legion of Super-Heroes, but multiple retcons have long since eliminated this possible future. Two of the most important rebels were Sinestro, the rogue Green Lantern, and Hal Jordan, driven insane over the loss of his home, Coast City, and possession by Parallax.
The Green Lantern Corps ranks were decimated by Hal Jordan during his madness, its power source, the Main Battery, extinguished; and Oa destroyed, removing the original power source for the rings.
After the fall of the Corps, other organizations tried to fill in the power vacuum left by the Guardians. Two organizations had initial notable successes; the Darkstars and L.E.G.I.O.N. However neither ever achieved the power and reach of the Green Lantern Corps.
Recently Oa was reconstructed by Hal Jordan's old sidekick Tom Kalmaku, and shortly thereafter Kyle Rayner, the last remaining Green Lantern, resurrected the Guardians as children using the power Parallax had stolen. The resurrections did not stop there. It was discovered that Hal Jordan had committed his crimes while possessed by the ancient fear parasite Parallax. Parallax had been imprisoned within the Central Power Battery for billions of years and is the mysterious "Yellow Impurity" within the Corps' green light that leaves them vulnerable against yellow. Once Parallax had been exorcised, the now adult Guardians re-imprisoned Parallax. With that, the Guardians continued the re-construction and expansion of the Corps. The new Corps, under the training of Kilowog, Kyle Rayner, Guy Gardner and other veterans, was primarily filled by new recruits. Only a handful of veteran Lanterns are back in the ranks. Each sector will have two Lanterns assigned to protect it. Due to an ancient pact with the Spider Guild the Vega star system is off limits to all members of the Corps.
With 3600 sectors and two Green Lanterns per sector, it will be many years before the Corps regains its former strength. It presently lacks both the manpower and the political influence it once had due to its years-long absence from many sectors. This has left it currently unable to intervene in situations it might have before Parallax destroyed the old Corps, including conflicts such as the Rann-Thanagar War. The Guardians remain staunchly uninvolved in what they see as a conflict that could destabilize many sectors, but that has not stopped Kyle Rayner and Kilowog from helping refugees.
Despite these difficulties, the Corps played a key role in defeating Superboy-Prime. Several Lanterns were killed slowing the renegade Superboy's advance on Oa, a sacrifice that enabled Earth's most powerful heroes to execute their plan to restrain him. Mogo, the sentient planet and Corps member, positioned himself to act as a final battleground between Superboy-Prime and the two Supermen. The Corps took responsibility for imprisoning Superboy-Prime, incarcerating him inside a small red Sun-Eater with fifty Green Lanterns on constant guard duty. "Prime Duty" is considered one of the lowlier functions of the Corps, and Guy Gardner was sentenced to a month of such duty for one of his frequent rules infractions.
As of the "One Year Later" timeframe, the Green Lantern Corps has increased its numbers, with many former trainees now full-fledged officers. The lack of truly experienced GLs remains an issue, with Guy Gardner being called upon often to assist the rookies. The transformation of Kyle Rayner to Ion also holds unknown ramifications for the future of the Corps.