The Pentagon and Reagan National Airport are located in Arlington County. Don't let the Washington, DC 20301-1400 postal address of the Pentagon fool you. It's west of Boundary Channel, which separates DC from Virginia, so the five-sided Department of Defense headquarters building is located in Virginia.
The Charles R. Fenwick Bridge carries Metro across the Potomac River from the District of Columbia. The name is appropriate not only because Fenwick advocated creating and funding the Washington Metropolitan Area Transit Administration while a State Senator from Northern Virginia. He was also a key negotiator in clarifying the boundary on the southern border of the river, separating the two jurisdictions - and ensuring that National Airport (now Reagan National) was clearly recognized as being in Virginia.