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 (which traditionally is a commonwealth itself) 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.