Adventures of Superman #444

In the second part of The Supergirl Saga, Superman mourns at the loss of Superboy's adoptive parents and learns about the sad state the Pocket Universe Earth is in following Superboy's disappearance from its reality. As told by its Lex Luthor, he was trying to find out where Superboy had disappeared to by using his equipment in Superboy's secret lab when he accidentally stumbled upon the three Phantom Zone criminals -- General Zod, Zaora, and Quex-Ul -- who claimed to be innocent survivors of the destruction of Krypton and let them loose from their prison. Destroying both the lab and the Phantom Zone projector, the Phantom Zone criminals went on a destructive rampage against the whole world, claiming to be its rulers and killing anyone that got in their way. Lex Luthor formed a resistance group and even gave Lana Lang powers similar to Superman, but it only resulted in the Phantom Zone criminals going so far as to destroy the Earth by burning away its atmosphere, killing everyone except those in Luthor's Smallville citadel. Learning the existence of the main New Earth universe, Luthor sent Supergirl across the dimensional barrier, blanking her memory but planting in her mind a compulsion to go to Metropolis in order to contact Superman. Superman at this point vows to join the resistance force to put an end to the Phantom Zone criminals' reign of terror on this world.