Zack Snyder’s Justice League has several visions of the future, but Cyborg’s vision reveals the outcome of Batman and Joker’s truce in the Knightmare future. The future of the world was fairly despondent in Justice League, with all the sequences of the Knightmare reality warning of a terrible outcome should the team fail in the present. During the epilogue of the film, Batman dreamed of this very future, one where he teamed up with enemies like the Joker and Deathstroke before being attacked by a Superman gone bad.
In Batman’s Knightmare vision, Joker called a truce. He offered a joker card to Bruce and told him that as long as he had it (and pending it was still whole), their truce would remain intact. The Crown Prince of Crime then taunted the Dark Knight about Robin’s death, which he would gladly discuss with Bruce should he think of ripping the card in half. Earlier in the film, Cyborg is plugged into Superman’s Kryptonian ship. He says it’s fearful of the Mother Box, which the Justice League is attempting to power up to resurrect Superman. Shortly after, Cyborg is shown the future, one where everything has gone to hell and Superman, now under the control of Darkseid, is wreaking havoc on the world. In this tragic future, Wonder Woman and Aquaman are dead, the Justice League’s headquarters destroyed, and Superman is seen holding Lois’ remains. Crucially, the joker card is torn as it flies through the air, suggesting Batman’s truce with Joker has come to an end.
It’s possible Batman worked with the Joker just long enough to send The Flash back in time to warn Bruce about the Knightmare future. Considering the chaos Joker has left in his wake and all the people he has killed, it’s likely Batman was only using Joker for a limited amount of time. If he got what he needed, then ripping the joker card in half makes a lot of sense. While Joker’s fate is ultimately unclear, there’s a chance Batman may have not only ended the truce, but killed the villain in retaliation for the death of Robin and so many others.
It’s unclear at what point in time the card was ripped in half, but it seems likely it was after the Knightmare scene in the epilogue. After all, considering how none of the future events had happened yet in Justice League, both Cyborg and Batman were only seeing pieces of the big picture. None of it was in chronological order, either, though the brief snippets were enough to cause concern. All that said, one can presume it was Batman who called off the truce since he was obviously in the position of power during the scene with the Joker.
It stands to reason Joker was at the end of his rope to bother calling a truce with Batman at all. However, the pair have a complicated, violent history and their working together was never going to last very long no matter how dire the circumstances of the desolate future. To be sure, the torn joker card is a small moment in Cyborg’s vision, but Justice League confirms that the Dark Knight and the Joker’s team-up is destined to end in one way or another.
