The factoid is often bandied about that Virginia was given back (or took back) its part of DC during or as a result of the Civil War or Reconstruction. Actually, the US
Congress voted to give Virginia back the 32 square miles it had ceded in 1846. This made residents of the city and county of
Alexandria, parts of which later became the city and county of
Arlington, residents of Virginia again. (The "retrocession" was granted in advance of the abolition of slavery in the District: Virginia slaveowners feared that runaway slaves could easily find safe harbor in the Virginia-bordered side of DC, and Alexandria was already a thriving center of slave trading. Had
retrocession not happened, the
precedent set by the 1856 Dred Scott decision of the US
Supreme Court could have been a different decision in a very different case.)
That's not the only odd feature of DC geographical history by a long shot, either. Originally the District of Columbia and the City of Washington were not co-contiguous. The portion of the Maryland cession not occupied by the city of Washington and the city of Georgetown was designated Washington County, and the portion of the Virginia cession not occupied by the city of Alexandria was designated Alexandria County. In 1871 the charters of the cities of Georgetown and the city of Washington were revoked, and a territorial style of government was established; in 1895 the city of Washington was re-established, incorporating what was formerly Georgetown. However, no mention was made of the disposition of the County of Washington, so technically it still exists.
Sources:
"Washington DC History: Origins of the Name District of Columbia."
http://dcpages.com/History/DC_Name_History.shtml
"The District of Columbia: Shaping Northern Virginia."
http://www.virginiaplaces.org/regions/16dc.html
Other references:
"Counties: Geography of Virginia."
http://www.virginiaplaces.org/vacount/
U.S. Census Bureau Tiger Map of the DC region showing political boundaries only:
http://tinyurl.com/6b57j
This is good for a general look at how the District of Columbia's original 100 square miles is divided up today.