In Flashpoint, Barry Allen goes back in time to rescue his mother from her killer, Eobard Thrawne, who's better known as the Reverse-Flash. It doesn't end well. Oh, Barry saves his mother just fine, but in the aftermath, reality itself is overwritten. In the new timeline, Bruce Wayne dies in an alley alongside his mother, forcing his father to take up Batman's mantle. Superman lives in a government lab, where he's kept in a permanently weakened state. Aquaman is a dictator who sank most of western Europe, the Justice League doesn't exist, and Wonder Woman is a straight-up supervillain.
She doesn't start that way, of course. As a young princess on Paradise Island, Diana yearns to see the world. One day, she sneaks onto a boat and heads out to sea, but doesn't get far before she's attacked by a sea monster. Thankfully, Aquaman is there to rescue her. Soon, the Amazons and Atlanteans have joined forces—and Diana and Arthur's wedding is supposed to solidify a permanent alliance between the two nations.
But during the ceremony, one of the Amazons murders Hippolyta, Diana's mother (she was aiming for Diana herself, but missed) and frames the Atlanteans. War follows. In a last-ditch effort to decimate Atlantis' forces, Wonder Woman and the Amazons destroy Paradise Island, killing thousands of soldiers, and flee to Britain. When they're denied asylum, they decide to stay anyway, rechristening the U.K. New Themyscira. Wonder Woman, now queen, kills Aquaman's lover Mera in a skirmish, and in response Atlantis floods Europe, murdering millions. Before long, the rest of the world is consumed by the Atlantean-Amazon conflict and the Flash is forced to go back in time and let his mother die in order to put everything back together.
It's very, very bad. And yet, it could've been even worse…