Acquired Cities:

Many times over the last two centuries, the United States has acquired land. A lot of the land was sparsely inhabited, but some contained cities. Name the city that was the most populous at the time of its acquisition.

(If the city became part of a Territory when it was acquired, and then part of a State, I'm counting the population at the time it became part of a Territory. Let's include for purposes of the question the land the U.S. "acquired" at the time it was created. And let's exclude land — such as Germany or Japan after World War II — that the U.S. merely ruled as a self-consciously temporary occupation. People who like to bring legal disputes even into geography problems should note that I'm assuming, together with Lincoln, that the seceding South never actually became foreign land, and thus wasn't reacquired in 1865.)

Here I don't have the precise statistics, but I'm pretty sure I know the right answer. I thought that, rather than just giving it, I'd ask people to post their thoughts on the Comments; naturally, if you have authoritative sources handy — or, better yet, linkable — you should post those. Later on (probably tomorrow), I'll post the right answer, together with an admission if it turns out that I got the answer wrong myself -- though as it happens, I've already posted my guess.