The DC Comics Universe was a mess. For years DC writers and artists had thrown continuity out the window. To make matters worse during the Silver Age the concept of alternate universes and Earths was used to maximum effect.
So many Supermen, Wonder Women, Batmen and other DC heroes and villains existed readers needed a score card just to keep track.
Meanwhile Marvel Comics', DC's chief competitor, sales numbers continued to increase. While DC's Universe lacked consistency and continuity, Marvel thrived on the fact that it had both.
Something had to be done.
Writer Marv Wolfman (along with penciller George Perez) pitched the idea of a massive event that would 'clean the slate' of the DC Universe and kick-start it with a whole new beginning.
DC execs agreed and in 1985 DC Comics' Crisis On Infinite Earth maxi-series was launched.
The series found the entire DC Universe in a state of flux. Entire alternate Earths and universes were destroyed by a devastating ant-force. Billions died, including many of DC's alternate heroes and villains.
The series tied into all of DC's titles and as Crisis progressed changes swept the DC Universe.
The first issue set up the 'Crisis' and introduced several key players: most notably the Monitor, Pariah, Harbinger and a mysterious dark force.
Later key DC heroes would die: the Flash and Supergirl among them.
Once finished the series would alter and condense the DC Universe into a manageable cohesive entity.