In August of 1970, the oldest alien entities of a distant galaxy apparently enlisted the aid of a master builder known as Creator2 into creating a planet to become the center of their universe. In order to do this, they would have to destroy the world where it was located and in its place create a replica of the planet Oa in the Earth-One* positive matter universe, and the planet Qward in the Anti-Matter universe. And that doomed world was Earth-Two!
In order to perform this dastardly deed, after determining that its denizens weren't worthy of continued existence, Creator2 captured and converted the android Red Tornado... making him a focal point to merge Earth-Two with its closest cosmic counterpart, Earth-One. This was possible because Red Tornado was a being of two universes, his android exterior and programming spawned from Earth-Two, and his inner cyclonic being and personality were from Earth-One's dimension.
With the two Earths occupying the same space in Earth-Two's universe, both would be destroyed. The resulting material and energy unleashed from this explosion would be the elements that Creator2 could use to create a Oa/Qward twin. The Justice Society thwarted this scheme, with Creator2 and his pawns perishing instead. Instrumental in their defeat and his world's preservation was Doctor Fate, and he could soon have to once more defend it when the aliens behind this amoral act appeared to finish the job...
Sometime later, Darkseid of the planet Apokolips attempted to shift his world from Earth-One's universe to where Earth-Two was located... thus bringing it into the center of that universe as its own version of Oa which he could rule. Many years later, during a possible future after the Crisis on Infinite Earths, Superboy of Earth-Prime and Alex Luthor of Earth-Three temporarily shifted the new sole universe so that instead of Oa, the space where Earth-Two had been now the center of the universe...to restore the Multiverse.
* The Weaponers of Qward created a bridge from their planet, at the center of the Anti-Matter Universe, to an alternative to the center of the prime positive universe, that being Earth-One. This space bridge would be used repeatedly by Hal Jordan aka Green Lantern and others from time to time.