Originally, Washington, D.C. was comprised of land given by the states of Maryland and Virginia, measured 100 square miles, and included the city of Alexandria. But in 1846, the federal government retroceded to Virginia the 31 square miles of land that had been taken from the state.
Several theories have been offered for why land was given back to Virginia, including that (1) the economy of Alexandria had stagnated, and giving the land back to Virginia would improve the economic prospects of the city; and (2) Alexandria had a slave market, and there was fear that abolitionists would push for the abolition of slavery in the District.
What I’ve not been able to determine is whether, during or after the civil war, Congress considered taking the land back from Virginia. Perhaps the federal government had no use for it, but Congress had no problem altering Virginia’s boundaries, as the recognition of West Virginia plainly shows.