Justice League Unlimited: The Return

Professor Ivo's robot Amazo destroys the planet Oa and defeats the Green Lantern Corps on his way to Earth to get revenge against a reformed Lex Luthor. The Justice League assembles its defenses while Luthor and The Atom use their nanotechnology knowledge to create a cannon to destroy the Amazo. It fails, but before Amazo can destroy him, Amazo returns Oa to the galaxy the robot hid it in and Luthor convinces Amazo to seek the knowledge coming with its evolution.