Justice League: Crisis on Two Earths was released on February 23, 2010, and is an adaptation of Justice League: World's Collide, an abandoned film project intended to be set in the DC Animated Universe which was to bridge together two television series, the ending Justice League and the commencing Justice League Unlimited.
This feature adapted from an original script called Justice League: Worlds Collide, which was designed as a film to bridge the gap between the Justice League and Justice League Unlimited television series. Originally, it was not used, but then returned to several years later. It was later re-worked, however, in order to make it stand alone and separate from the DC Animated Universe.
The overall story was originally adapted from the many Crime Syndicate stories which have been published by DC Comics, such as Gardner Fox's "Crisis On Earth Three!" from 1964 and Grant Morrison's JLA: Earth 2, from 2000.