When the Imperium invaded the Earth, J'onn Jonzz of Mars, who was assigned to keep them imprisoned on his world after they devastated his, was captured by the US Army. To find help, he telepathically contacted other individuals including Superman and Batman. They would immediately be joined by Green Lantern, Hawkgirl of Thanagar, the Flash, and newcomer Diana of Themyscira. After apprehending the threat, the seven would form a superhero team dubbed the Justice League.
After Hawkgirl betrayed the Earth for Thanagar's conquest (which ultimately failed due to the efforts of the League) and the subsequent loss of the Watchtower I, the team reformed without the self-exiled Thanagarian in addition to expanding the roster to more superheroes, as well as reconstructing a new Watchtower.