World Cities A-Z: Country with biggest second largest City
List all cities that are their country's 2nd largest by initial letter, you'll find that the largest starts with S: can you identify the country and its two largest cities beginning with S?
Now remove all cities from that country or starting with S and to the same: you'll find the largest starts with D. Keep going!
Data from citypopulation.de
Only cities with at least 1M population are considered, so not all letters are included