In Batman v Superman: Dawn of Justice, it was established that The Joker had killed Robin. That lines up with what we've seen in the comic books, of course, as the Clown Prince of Crime murdered Jason Todd in cold blood, prompting the Dark Knight to display his sidekick's suit in the Batcave.
Suicide Squad put The Joker front and centre, but didn't really have him interact with Batman beyond a quick chase sequence that was more about Harley Quinn than it was the Harlequin of Hate!
During the closing moments of Zack Snyder's Justice League, we returned to the post-apocalyptic world that had been decimated by Darkseid and Superman. However, much to all of our surprise, the Caped Crusader was working with The Joker to try and put an end to the Man of Steel's tyrannical reign over Earth.
Taking at his "Full Circle" even this weekend, Snyder finally offered an explanation about what brought the two enemies together.
"Our theory is that [the Joker is] the one who knows where the Kryptonite, what shards of Kryptonite exist," the director explained. "He has that information. And so he's being carried...he has basically made a deal with Batman. 'Don't kill me, and I'll show you. I'll get you a tool to fight Superman.'"
"And so [Batman] is kind of stuck with him, in that way. If he kills him, or gets rid of him, then he's screwed himself."
As for what the villain actually did to Robin, Snyder refused to share too many details in case he gets to tell the story down the line, but did say, "At that point, [Batman’s] real vulnerability was Robin. What’s obvious is that the Joker understood that. Clearly, he blew him up. Or, it looks like he burned or scorched him."
We don't anticipate Snyder getting to make his Justice League sequels unless it's as an animated movie or comic book, though it's hard not to think about how much fun it might have been to see this story play out in its entirety!
Do you think a Batman/Joker team-up was the way to go in the DCEU?