Near-complete nuclear destruction, to put it bluntly.

[Small detail about his world, really. Just a little footnote.]

Mankind's obsession with technology outpaced its restraint, its willingness to use it responsibly. Greed became the catalyst for an escalation of tensions, then war, then the day that planet was nearly annihilated by atomic bombs. Only sheer luck or a miracle prevented humans from going extinct as a species, and two hundred years later, we're still trying to pick up the pieces.