Qing Dynasty - context
• In 1400—that is to say, during the Great Ming empire—Nanjing was the largest city in the world in terms of population. The second biggest was Vijayanagar in India, followed by Cairo, then Paris [Robert B. Marks]. In the second half of the 17th Century—that is to say, at the outset of the Great Qing empire—it was far from its earlier size, possibly being the "emptyist" city in the world.
As the Manchus drove southward to unify their prize, and seek out and kill all princes-of-the-blood so as to eliminate any potential claimants to the throne, townspeople—particularly those in cities such as Nanjing that had been home to such princes—fled in fear before the invaders. The city didn't regain its earlier size for decades to come.