In "The Stone Dogs," Eric von Shrakenberg refers to Archona as "Earth's greatest city" and describes it in detail, discussing its 12 million people, the nuclear reactors, its industrial/post-industrial development, etc.
So how might we get it so Atlanta, Georgia is "Earth's greatest city" in the equivalent timeframe (IIRC 1980 or so)?
No nuclear wars knocking off more well-known competitors, please.
This is in post-1900 because of the timeframe, but the POD can be anytime.