In 1963, DC Comics began an annual crossover story involving the Justice League of Earth-One and the Justice Society of Earth-Two.
It was called “Crisis on Earth-[fill in the blank]”
Details here.
Earth-Prime was a favorite of mine.
In 1985, DC published the last JLA/JSA crossover.
“Crisis on Infinite Earths” was DC’s first attempt at streamlining and consolidating their timeline by eliminating the multiverse. They’ve since backtracked and went from a single universe to a multiverse again.
Somewhere along the line they had a “Crisis on Earth-X” story.
And now they’ve adapted it to the CW’s Arrowverse.
So far, it’s awesome.
So much better than last year’s “Invasion!”