The atomic wars of the planet Mega were a series of nuclear wars that devastated the planet. According to Superman, no life on the entire planet survived the wars. He was in error of course, because one man did survive the conflicts, a man named Hecla. (it was also later learned that a few other Coluans had survived as well) He then built an android named Brainiac, and sent him into space to find specimens from other worlds to repopulate the mostly uninhabited planet. That's how Brainiac made his way to Earth, to collect Earth specimens and take them back to Mega.
The state of Mega directly following the atomic wars was unknown, but it can be assumed that most of the planet was uninhabitable due to atomic radiation.