United States Department of Defense

The Space Development Agency (SDA) is a directorate of the United States Department of Defense's office of the under secretary of defense for research and engineering (OUSD (R&E)) which supports space development in the interests of U.S. national security. By design, the functions for acquisition and sustainment (A&S) are the responsibility of another under secretary of defense; this separation of function decouples the technology development of a working prototype system, even the systems as complicated as those taken on by the SDA, from overcomplication induced by the processes of the DoD. The SDA was created in 2019 to implement the National Defense Space Architecture (NDSA). The SDA plans to transfer to the United States Space Force around October 2022, although details are still uncertain.

The National Military Command Center (NMCC) is a Pentagon command and communications center for the National Command Authority (i.e., the President of the United States and the United States Secretary of Defense). Maintained by the Department of the Air Force as the "DoD Executive Agent" for NMCC logistical, budgetary, facility and systems support; the NMCC operators are in the Joint Staff's J-3 (Operations) Directorate. "The NMCC is responsible for generating Emergency Action Messages (EAMs) to missile launch control centers, nuclear submarines, recon aircraft and battlefield commanders".

The National Military Command System (NMCS) was the federal government of the United States' Cold War command and control system that consisted of the National Military Command Center (NMCC) at The Pentagon, the Alternate National Military Command Center (NMCC) at Pennsylvania Raven Rock Mountain, 3 National Emergency Airborne Command Post (NEACP) aircraft on 24-hour ground alert, 2 National Emergency Command Post Afloat (NECPA) ships, "and interconnecting communications".

The Defense POW/MIA Accounting Agency (DPAA) is an agency within the U.S. Department of Defense whose mission is to recover American military personnel listed as prisoners of war (POW) or missing in action (MIA) from designated past conflicts, from countries around the world.

The Pentagon is the headquarters building of the United States Department of Defense. It was constructed on an accelerated schedule during World War II. As a symbol of the U.S. military, the phrase The Pentagon is often used as a metonym for the Department of Defense and its leadership. Located in Arlington County, Virginia, across the Potomac River from Washington, D.C., the building was designed by American architect George Bergstrom and built by contractor John McShain. Ground was broken on 11 September 1941, and the building was dedicated on 15 January 1943. General Brehon Somervell provided the major impetus to gain Congressional approval for the project; Colonel Leslie Groves was responsible for overseeing the project for the U.S. Army Corps of Engineers, which supervised it. The Pentagon is the world's largest office building, with about 6.5×10^6 sq ft (150 acres; 60 ha) of floor space, of which 3.7×10^6 sq ft (85 acres; 34 ha) are used as offices. Some 23,000 military and civilian employees, and another 3,000 non-defense support personnel, work in the Pentagon. It has five sides, five floors above ground, two basement levels, and five ring corridors per floor with a total of 17.5 mi (28.2 km) of corridors. The central five-acre (2.0 ha) pentagonal plaza is nicknamed "ground zero" on the presumption that it would be a prime target in a nuclear war.In 2001, the Pentagon was damaged during the September 11 attacks. Five al-Qaeda hijackers flew American Airlines Flight 77 into the western side of the building, killing themselves and 184 others: 59 on the airplane and 125 in the Pentagon. It was the first significant foreign attack on Washington's governmental facilities since the city was burned by the British during the War of 1812. Following the attacks, the western side of the building was repaired, with a small indoor memorial and chapel added at the point of impact. An outdoor memorial dedicated to the Pentagon victims of 9/11 opened in 2008.

Pentagon Army Heliport (ICAO: KJPN, FAA LID: JPN) is a military heliport serving the Pentagon. It consists of a single pentagon-shaped helipad and is located on the northern side of the Pentagon building. It is used for ferrying VIPs such as military leaders and foreign guests to and from the Pentagon by helicopter.

American Airlines Flight 77 was a scheduled American Airlines domestic transcontinental passenger flight from Washington Dulles International Airport in Dulles, Virginia, to Los Angeles International Airport in Los Angeles, California. The Boeing 757-223 aircraft serving the flight was hijacked by five Saudi men affiliated with al-Qaeda on the morning of September 11, 2001, as part of the September 11 attacks. They deliberately crashed the plane into the Pentagon in Arlington County, Virginia, near Washington, D.C., killing all 64 aboard (including six crew and themselves) and another 125 in the building. Less than 35 minutes into the flight, the hijackers stormed the cockpit and forced the passengers, crew, and pilots to the rear of the aircraft. Hani Hanjour, one of the hijackers who was trained as a pilot, assumed control of the flight. Unknown to the hijackers, passengers aboard made telephone calls to friends and family and relayed information on the hijacking. The hijackers crashed the aircraft into the western side of the Pentagon at 09:37 AM ET. Many people witnessed the crash, and news sources began reporting on the incident within minutes. The impact severely damaged an area of the Pentagon and caused a large fire that took several days to extinguish. A portion of the building collapsed. The damaged sections of the Pentagon were rebuilt in 2002, with occupants moving back into the completed areas that August. The 184 victims of the attack are memorialized in the Pentagon Memorial adjacent to the crash site. The 1.93-acre (7,800 m2) park contains a bench for each of the victims, arranged according to their year of birth, ranging from 1930 to 1998.

Pentagon Transit Center is a split platform station on the Washington Metro located adjacent to The Pentagon in Arlington, Virginia. The station was opened on July 1, 1977, and is operated by the Washington Metropolitan Area Transit Authority. Providing service for both the Blue and Yellow Lines, the station is where the two lines diverge and thus acts as a transfer point. Northbound, the Blue Line continues through Virginia and the Yellow Line crosses the Potomac River into the District of Columbia. The station opened on July 1, 1977 with the completion of 11.8 miles (19.0 km) of rail between National Airport and RFK Stadium.

The Pentagon Memorial, located just southwest of The Pentagon in Arlington County, Virginia, is a permanent outdoor memorial to the 184 people who died as victims in the building and on American Airlines Flight 77 during the September 11 attacks.Designed by Julie Beckman and Keith Kaseman of the architectural firm of Kaseman Beckman Advanced Strategies with engineers Buro Happold, the memorial opened on September 11, 2008, seven years after the attack.

The Victims of Terrorist Attack on the Pentagon Memorial is a memorial over a group burial site at Arlington National Cemetery in the United States. It commemorates the victims of the attack on the Pentagon, which was struck by a Boeing 757 commercial airliner hijacked by five al-Qaeda terrorists on September 11, 2001, killing 184 people (excluding the hijackers). The memorial specifically honors the five individuals for whom no identifiable remains were found. However, a portion of the remains of 25 other victims are buried at the site. The names of the 115 Pentagon employees and 10 contractors in the building, as well as the 53 passengers and six crew members aboard American Airlines Flight 77 (which crashed into the building) are inscribed on the memorial.

Washington Airport was the second major airport to serve the city of Washington, D.C., in the United States. Located in Arlington, Virginia, near the intersection of the Highway Bridge and the Mount Vernon Parkway (where The Pentagon's south parking lots, Metrobus bus bays, and a portion of Interstate-395 now exist). The first airport to serve the city was Hoover Field, a private airfield constructed in 1925. Washington Airport, a private airport triple the size of Hoover Field, was built literally across the road in late 1927. The airfield suffered from short and unpaved runways, numerous life-threatening obstructions around the field, poor visibility (due to a burning garbage dump adjacent to each field), and poor drainage. Washington Airport nearly went bankrupt in 1933, and it was auctioned off to a new owner. The new owner also owned Hoover Field, and merged the two into a single airfield, Washington-Hoover Airport. Washington-Hoover Airport closed in 1941 when Washington National Airport (now Ronald Reagan Washington National Airport), a replacement facility, was opened.

Hoover Field was an early airport serving the city of Washington, D.C. It was constructed as a private airfield in 1925, but opened to public commercial use on July 16, 1926. It was located in Arlington, Virginia, near the intersection of the Highway Bridge and the Mount Vernon Memorial Parkway, where The Pentagon and its northern parking lots now stand.Considered one of the most hazardous airfields in the United States, Hoover Field suffered from short and unpaved runways, numerous life-threatening obstructions around the field, poor visibility (due to a burning garbage dump to its northwest), and poor drainage. It was purchased by the owner of nearby Washington Airport in early 1929, causing a brief merger of the two fields, but was sold to a new owner just 12 months later. It nearly went bankrupt in 1933, and was sold at auction and merged with Washington Airport to become Washington-Hoover Airport on August 2, 1933. Washington-Hoover Airport closed in June 1941. Washington National Airport (now Ronald Reagan Washington National Airport) was built as its replacement.