Arlington is a residential suburb of Washington, DC, located across the Potomac in Virginia. Technically, it is not a municipality but a county, originally included in the District of Columbia but returned to Virginia in 1846. Around 1740, Gerard Alexand built Abingdon, the first great mansion within the present limits of Arlington. The site of the ruins has been restored on the grounds of Reagan International Airport. John Ball was one of the earliest settlers in what is now Arlington County. He obtained a 166-acre grant in 1742 and built a one-room log cabin. The cabin has survived two and a half centuries and is now owned by the Arlington Historical Society, who call it the Ball-Sellers House after its first and last private owners. The original name was Alexandria County. The original District of Columbia was ten miles square, but this portion was considered to be excess to the needs of the federal government, so Congress retroceded it to Virginia. During the Civil War, Union forces quickly occupied it and built some 20 forts for the defense of Washington. By the end of the war, the area was in shambles and few of the pre-war residents remained. In 1870, Alexandria City was separated from Alexandria County. By the turn of the century, unsavory elements had become established in the county, so law-abiding citizens took steps to install a reform government for the county. The extension of trolley lines and the Washington & Old Dominion Railway helped the county grow as a residential suburb of Washington. In 1920, in order to avoid confusion with the city of Alexandria, the county's name was changed to Arlington, in recognition of the home of Robert E. Lee, which is on the Arlington National Cemetery reservation. Arlington is the site of the Pentagon, built at the start of World War II, headquarters of the United States Department of Defense and the largest office building in the world. The Arlington Historical Museum occupies the former Hume School, built in 1891 and the oldest school building in Arlington. Although Arlington is in Virginia, part of the South, it was an early adopter of school integration and admitted the first black students in 1959. The Newseum takes visitors behind the scenes to see how the news is made. The Drug Enforcement Administration runs the DEA Museum and Visitor Center in Pentagon City.