Justice League (DCAU)

History
Following Darkseid's invasion of Earth, Superman regained the majority of the world's trust after being brainwashed and used as a pawn. Though doubts remained regarding the Man of Steel's reliability, none put more faith in the hero than Senator J. Allen Carter. Carter proposed world peace through weapons disarmament, and nominated Superman for the job. Though skeptics aired their grievances, the World Assembly passed the motion and Superman set to work.

As Superman finished disarming the weapons of destruction, meteors rained down upon Earth. From the wreckage, massive alien creatures appeared, and without hesitation made their intentions clear: conquest. Superman and Batman rushed to the scene, throwing everything within their arsenals at the creatures but their efforts seemed in vain.

As the world panicked about the newest alien invasion, Superman fled the scene. Batman tracked the hero to a US Government base. Investigating further, the two discovered a different alien held prisoner there. Freeing him, the man introduced himself as J'onn J'onzz, but the newly-formed trio quickly found themselves under attack by the Imperium. The trio was still outnumbered, but not for long. At a critical moment, the Green Lantern John Stewart, Hawkgirl, Wonder Woman, and the Flash arrived on the scene. The Imperium forces fought back, but they lost; Earth's heroes had assembled.

J'onn explained the Imperium Invasion. The septet then broke into three groups, each set on destroying the Imperium factories created on Earth. The heroes fought valiantly, but to no avail. The Flash and Green Lantern made a hasty retreat, while Superman and Hawkgirl were captured. J'onn, Batman, and Diana discovered a method to defeat the Imperium, but in the end only Diana and J'onn escaped. The four remaining heroes regrouped at Metropolis, and prepared to free Hawkgirl and Superman. Keeping a relatively low profile, the heroes managed to subdue the few Imperium forces to cross their path, but eventually were captured as well.

Reunited with their friends (albeit via captivity), the heroes were shocked to discover the Imperium organizer to be none other than Senator Carter. The real J. Allen Carter had perished on Mars during a space expedition. Using his persona to their advantage, the Imperium tricked the world into disarming itself. The Earth disarmed, no obstacles remained to prevent their invasion. Earth's heroes, however, proved too resilient. Batman freed the other six, and the group destroyed factories and armada.

In the aftermath of the invasion, Batman built and the activated Watchtower, set to orbit around the Earth to act as an early warning system. The heroes recognized the Earth's need for protection, and the Justice League formed.

(Taken from the DC Animated wikia