I can just imagine the look that Kate Masur’s graduate school overlords gave her when she told them she wanted to study the Civil War history of Washington, D.C.

Kate, Kate, Kate, their arched eyebrows must have said. Washington? Really?

“There was a lot of skepticism among my advisers,” Kate said.

Nobody had done much research on the topic. It was thought there wouldn’t be many sources to consult. And even if there were, what was the point? Washington, the thinking goes in some academic circles, is weird: a capital that’s more than a city but less than a state....