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Zack Snyder’s Justice League ending is full of potential for the DCEU, and sets up a Justice League 2 and 3 and more sequels that we may unfortunately never get to see.

And thats where Flash comes in. As Barry begins building up the speed necessary to generate the kind of power required for this, hes hit by a lucky shot from a Parademon, delaying Cyborgs ability to prevent Unity from happening. In the process, the Mother Boxes unleash a torrent of energy powerful enough to vaporize everything and everyone in its path (and possibly the entire world), including Wonder Woman, Cyborg, Aquaman, and even Superman. Barry witnesses this from afar and with only nanoseconds to act, decides to break his own rule about not surpassing the speed of light, for fear of the crazy things that happen to time he mentioned earlier in the film. 
Needless to say, Barry succeeds in his goal and seems to reverse time by the necessary few seconds as he approaches the Mother Boxes, reconstituting everyone, delaying the explosion long enough for him to give Cyborg the energy needed to hack the Mother Boxes, and stopping things in their tracks long enough for Aquaman, Superman, and Wonder Woman to send Steppenwolf back home in pieces.
Were going to come back to Barry in a minute, but lets just take a moment here to talk about Steppenwolfs boss.
Or he would have returned if Zack Snyders original plan for multiple Justice League movies had come to pass. When I made the film originally, it was part of a five-part trilogy, the director recently told Vanity Fair. There were two more episodes of the Justice League to be shot.
Darkseid isnt cowed witnessing Steppenwolf have the extraterrestrial crap beaten out of him by Superman, skewered by Aquaman, and decapitated by Wonder Woman. Instead he calmly tells DeSaad to ready the armada, well use the old ways, indicating that hes ready to just invade Earth the old-fashioned way again with a massive alien army and warships. 
Why he feels confident in doing this considering how it ended for him the last time is anyones guess, but the fact that he knows for sure that Anti-Life is here probably has something to do with it, along with the fact that he can use the Anti-Life Equation to control Superman, who just proved himself to be the most powerful champion in the galaxy.