In the world of comic books, there are few rivalries as heated as Batman and the Joker. The two of them couldn't be more different. Yet, within the past 75 years of their existence, there have been moments when the Joker and Batman join forces.
As unbelievable as it sounds, it's true. There are a handful of times where Batman and the Joker work together. They usually don't like it. Hell, they're within inches of killing each other most of the time, but there are some circumstances where Batman teams up with the Joker to achieve a common goal.
While it is rare when it happens, whenever the Dark Knight partners up with the Clown Prince, their opposition is royally screwed. Check out these odd Batman-Joker collaborations in comic book history.
When They Took Down the Red Skull
In the Batman & Captain America crossover, it's natural that the two heroes' greatest foes would also team up in the book. However, when the Joker teamed with the Red Skull, he didn't know what he was signing up for. In an odd bout of patriotism, Joker turns on the Red Skull to help Batman and Cap take him down after finding out the Red Skull was working in honor of the Third Reich. Why this change of heart? Because while the Joker is an insane, remorseless killer that takes pleasure in the torment of others, he draws the line at being a Nazi.
When Brother Eye Merged Them
In The New 52: Futures End story, the super-intelligent AI known as Brother Eye somehow merged the Joker and Batman together to form a superbeing that he could control. Batman's high intellect and the Joker's malice combined to form a nearly unstoppable "Jokerborg," as he was called. Luckily, Terry McGinnis, a.k.a. Batman Beyond, was able to dismantle this abomination.
When Batman Raised the Joker from the Dead
In Batman: Legends of the Dark Knight #145, the Joker and fellow Batman foe Ra's Al Ghul had a friendly game of chess and were thinking of doing their own team-up. Too bad the Clown Prince of Crime didn't fit into the plans and Talia Al Ghul shot him.
In a desperate attempt to find Ra's and stop his scheme, Batman tossed the Joker's body into the Lazarus Pit. Traditionally, the Lazarus Pit brings the dead back to life, but causes the resurrected to become insane. However, since the Joker was insane to begin with, his resurrection fixed his mind. He helped Batman find Ra's and stop his evil plot, all the while pining in remorse over the countless murders he had committed in the past.
When They Fought the Punisher
In the crossover Punisher/Batman: Deadly Knights, the Joker and Batman team up (albeit briefly) to stop Frank Castle. Castle is about to shoot the Joker point-blank in the head, but Batman intervenes. As the two street vigilantes tussle, Batman actually tells the Joker to run and make an escape. The fact that Batman did such a thing for a sworn enemy shows how committed he is to his "no killing" code.