20th century

In 1920, the Virginia legislature renamed the area Arlington County to avoid confusion with the City of Alexandria which had become an independent city in 1870 under the new Virginia Constitution adopted after the Civil War.
In the 1930s, Hoover Field was established on the present site of the Pentagon; in that decade, Buckingham, Colonial Village, and other apartment communities also opened. World War II brought a boom to the county, but one that could not be met by new construction due to rationing imposed by the war effort.
In October 1942, not a single rental unit was available in the county.^[25] The Henry G. Shirley Highway (now Interstate 395) was constructed during World War II, along with adjacent developments such as Shirlington, Fairlington, and Parkfairfax.

Netherlands Carillon

In February 1959, Arlington County Schools desegregated racially at Stratford Junior High School (now H-B Woodlawn) with the admission of black pupils Donald Deskins, Michael Jones, Lance Newman, and Gloria Thompson. The U.S. Supreme Court's ruling in 1954, Brown v. Board of Education of Topeka, Kansas had struck down the previous ruling on racial segregation Plessy v. Ferguson that held that facilities could be racially "separate but equal." Brown v. Board of Education ruled that "racially separate educational facilities were inherently unequal." The elected Arlington County School Board presumed that the state would defer to localities and in January 1956 announced plans to integrate Arlington schools. The state responded by suspending the county's right to an elected school board. The Arlington County Board, the ruling body for the county, appointed conservatives to the school board and blocked plans for desegregation. Lawyers for the local chapter of the National Association for the Advancement of Colored People (NAACP) filed suit on behalf of a group of parents of both white and black students to end segregation. Black pupils were still denied admission to white schools, but the lawsuit went before the U.S. District Court, which ruled that Arlington schools were to be desegregated by the 1958–59 academic year. In January 1959 both the U.S. District Court and the Virginia Supreme Court had ruled against the Commonwealth of Virginia's massive resistance movement, which opposed racial integration.^[26] The Arlington County Central Library's collections includes written materials as well as accounts in its Oral History Project of the desegregation struggle in the county.^[27]

The former Navy Annex and the Air Force Memorial

Arlington during the 1960s was undergoing tremendous change after the huge influx of newcomers in the 1950s. The old commercial districts did not have ample off-street parking and many shoppers were taking their business to new commercial centers, such as Parkington and Seven Corners. Suburbs further out in Virginia and Maryland were expanding, and Arlington's main commercial center in Clarendon was declining, similar to what happened in other downtown centers. With the growth of these other suburbs, some planners and politicians pushed for highway expansion. The Federal Aid Highway Act of 1956 would have enabled that expansion in Arlington. However, the administrator of the National Capital Transportation Agency, economist C. Darwin Stolzenbach, saw the benefits of rapid transit for the region and oversaw plans for a below ground rapid transit system, now the Washington Metro, which included two lines in Arlington. Initial plans called for what became the Orange Line to parallel I-66, which would have mainly benefited Fairfax County. Arlington County officials called for the stations in Arlington to be placed along the decaying commercial corridor between Rosslyn and Ballston that included Clarendon. A new regional transportation planning entity was formed, the Washington, Metropolitan Transit Authority. Arlington officials renewed their push for a route that benefited the commercial corridor along Wilson Boulevard, which prevailed. There were neighborhood concerns that there would be high density development along the corridor that would disrupt the character of old neighborhoods. With population in the county declining, political leaders saw economic development as a long range benefit. Citizen input and county planners came up with a workable compromise, with some limits on development. The two lines in Arlington were inaugurated in 1977. The Orange Line's creation was more problematic than the Blue Line's. The Blue Line served the Pentagon and National Airport, and boosted the commercial development of Crystal City and Pentagon City. Property values along the Metro lines increased significantly for both residential and commercial property. The transformation of Clarendon is particularly striking, with its transformation from a downtown shopping area, ensuing decay, home to a vibrant Vietnamese business community in the 1970s and 1980s known as Little Saigon, and now is a vibrant urban village. Arlington's careful planning for the Metro has transformed the county and has become a model revitalization for older suburbs.^[28][29]

21st century

The Pentagon being reconstructed in February 2002, after the September 11 attacks.

On September 11, 2001, five al-Qaeda hijackers deliberately crashed American Airlines Flight 77 into the Pentagon, killing 115 Pentagon employees and 10 contractors in the building, as well as all 53 passengers, six crew members, and five hijackers on board the aircraft.
In 2009, the construction of Turnberry Tower, the tallest residential building in the Washington metropolitan area, was completed in the Rosslyn neighborhood.^[30]
1812 N Moore in Rosslyn, completed in 2014, is the tallest building in the Washington metropolitan area, exceeding the nearby Rosslyn Twin Towers that were constructed three decades before it.