The Justice League of America was one of the world's premiere super teams alongside other teams such as the Avengers. They saved the world countless times from threats that most people could never imagine. When the original team died at the hands of the Chrell it was only a matter of time before others rose to take their place. The new team, taking the name Justice League, has sworn to protect not just America, but the world from injustice.
Nearly 20 years after the Chrell Invasion that literally killed off all the heroes on the planet Earth, the new Superman, Ree-Van, spent those decades slowly rebuilding the ruined hulk of the Justice League's old Watchtower, which was all but destroyed in the Chrell's intial invasion.
Currently Superman is reconnecting with the new heroes that are beginning to emerge and have taken up the mantles of the fallen heroes of the Chrell Invasion. Since there has already been an incursion by Darkseid himself, it is imperetive that the Justice League be reborn to combat him, or others like him, should they re-emerge.