Columbia Commonwealth

In the early 21st century, when the United States was divided into thirteen commonwealths, Washington, DC, the pre-War capital city of the U.S. became part of the Columbia Commonwealth, along with Virginia and Maryland.

Not to be mistaken for the Commonwealth that Dr. Zimmer hails from.

Trivia
Bethesda Game Studios, the developer of Fallout 3, is headquartered in real-life Rockville, Maryland, although the name comes from Bethesda, Maryland, where the company was originally founded in 1985.