Arlington County is a county containing about 210,000 residents in the Commonwealth of Virginia. It is located directly across the Potomac River from Washington, D.C., and the two of them are linked by several highway and railroad bridges, and a tunnel beneath the river. A major airport of Washington, D.C., Ronald Reagan Washington National Airport, many Federal Government offices, including those of the Department of Defense, the Department of the Navy, the Drug Enforcement Agency, and Arlington National Cemetery are located in Arlington County.The county was originally founded as Alexandria County on February 27, 1801 by passage of the District of Columbia Organic Act of 1801. Alexandria County along with the now-defunct Washington County were the two counties formed on that date within the District of Columbia. Due to issues involving congressional representation, abolition and economic decline, the county along with the then-Town of Alexandria were returned to the Commonwealth of Virginia in an act of the U.S. Congress which took effect in 1847. In 1920 the General Assembly of Virginia changed the name of the county to "Arlington County" in order to avoid confusion with Alexandria, which had become an independent city in 1870.Arlington County is organized legally as one of the counties of Virginia. For purely statistical considerations, it is included with the nearby cities and counties as a city of the Washington Metropolitan Area by the U.S. Census Bureau. With a land area of 26 square miles (67 km2), Arlington County is geographically the smallest self-governing county in the United States.In 2005, Arlington was ranked first among walkable cities in the United States by the American Podiatric Medical Association.Arlington is the location of the Arlington National Cemetery, the Washington National Airport, The Pentagon (of the U.S. Department of Defense), Fort Myer (of the U.S. Army), the Pentagon Memorial, the U.S. Marine Corps Memorial, and the U.S. Air Force Memorial.