A thousand years ago, on the planet Krypton, there was a technologically advanced, cruel and nihilistic race which carved a vast stellar empire, subjugating thousands of civilisations under their regime. Only one planet managed to overthrow the grip of the empire, Oa, but their rebellion only served to free their own planet.
Their thirst for power eventually cost the destruction of their own homeworld. In the dying moments, one of their top scientists sent his offspring to one of their genetic stock planets in the hope that he would revenge them by breeding a new race of soldiers. The spacecraft travelled for a thousand years until it reached the small blue marble orbiting a yellow sun. The primitive present on the planet were seeded millions of years before and would be almost completely genetically compatible with the offspring. The scientist had not realised that these primitives would had evolved rapidly in their culture in the intervening years. The Kryptonian offspring would fail in his mission, living as one of the primitives, becoming their protector.
But the offspring was not the only survivor. Thousands of their genetically superior supersoldiers had survived the destruction of the planet and were now warlords over the planets they had subjugated reporting to a military command structure engineered to survive a cataclysm - their commanders reside on Kandor, previously a planet not dissimilar to Earth, whose population were also seeded by the Kryptonian war machine.
The rebellion would be slow, it would be thorough and it originated from Oa. Their agents would spread through the shattered Kryptonian empires and recruit forces of rebellion from their own people. Oan scientists had discovered that fragments of the destroyed planet were toxic to the survivors. Each type would cause a different effect, but only one could be used as more than a poison, only one would be able to take the fight to the stars: green kryptonite.
And from these fragments, they fashioned their weapons.