"Batman v Superman: Dawn of Justice," out Friday, may confuse those who think the Dark Knight and the Man of Steel as best friends. The strange thing is, the pair have been BFFs for much of their published history — which hasn't prevented them from coming to blows an embarrassing number of times.

How did these Spandex superstars become super frenemies? Weirdly, it began with the 1939 New York World's Fair. When the Fair opened 77 years ago, National Periodical Publications — known today as DC Comics — issued a 96-page anthology for 15 cents. Superman appeared on the cover. By 1940, DC had another superstar, Batman. In the second issue of "World's Fair Comics," the two shared the cover.

In the 1950s, the two started appearing in stories together on a more regular basis — and they often fought (like brothers). But comics, like people, grew up. By the 1980s the stories in "World's Finest" had gotten a lot more serious, and sometimes it seemed like Batman and Superman were fighting for real. It all came to a head in 1986, with the publication of two series that would change DC Comics forever.

The first was "Crisis on Infinite Earths," a 12-issue series that relaunched DC's major heroes. In the new DC Universe, polar opposites Batman and Superman didn't get along. Batman thought Superman too much of a Boy Scout. Superman thought Batman's modus operandi was borderline criminal.

The other major book was "Dark Knight Returns," by Frank Miller. Miller's story was set in a dystopian future in which Superman is a tool of the government sent to stop an aging Batman, who is making one last crusade to save his city. This is the story that will likely influence "Batman v Superman," in that the Dark Knight shows up for the Super-rumble in an armored suit powered by kryptonite. You have to read the book to find out how that ends.

Miller's story seems to have influenced every Superman/Batman story since. Some others include:

An interior page from “Dark Knight Returns.”

• "Red Son," an "Elseworlds" tale in which Superman's baby rocket lands in the Soviet Union. The Soviet-trained Superman grows up to succeed Josef Stalin and try to impose a workers' paradise on Earth. But he keeps getting harassed by an underground terrorist witnesses say looks like a giant bat.

• "Batman: Hush" is a story that incorporates most of Batman's friends and foes, including Superman, who ends up being both when Poison Ivy controls his mind.

• "Justice League: Origin" is the result of yet another reboot of the DC Universe, with Batman and Superman meeting again for the first time. This time the Dark Knight isn't prepared for a Kryptonian, and neither he nor Green Lantern can slow the Man of Tomorrow down for long.

And if those stories and the movie don't quench your thirst for Batman vs. Superman stories, don't worry — there are plenty more.