Washington, D.C. is the capital city of the United States of America. D.C. stands for District of Columbia. It is a square-shaped region on the Potomac River between the states of Virginia (to the west) and Maryland (to the east).
In Watchmen, Washington, D.C. is significant as the location where Silk Spectre and Dr. Manhattan attempted to control angry demonstrators outside the grounds of the White House during the Police Strike of 1977. Juspeczyk had managed to separate the crowd's leaders; however, fearing that they would soon become violent, Dr. Manhattan teleported the entire crowd to their individual homes.
Washington, D.C. at Wikipedia