Constructed as the United States mobilized before the Second World War, the Pentagon Building was, at the time, the largest office complex in the world. It remains the headquarters of the Department of Defense and the senior officials of the United States armed services, but the Department long ago exceeded the space inside the facility.
It does contain the most senior officers of the Office of the Secretary of Defense; the Organization of the Joint Chiefs of Staff; the civilian Departments of the Army, Navy and Air Force; and the immediate offices of the Chief of Staff of the Army, Chief of Naval Operations, and Commandant of the Marine Corps.
In the Pentagon is the National Military Command Center, which, while manned continuously, does not directly control troops. The NMCC does manage information flow between the National Command Authority and the Unified Combatant Commands. While it has modern workstations, the War Room beloved of movies such as Dr. Strangelove has very little resemblance to the rather prosaic facilities. Called "the Tank", the Joint Chiefs of Staffs' conference room looks much as does many budget-conscious corporate offices.
Authorization, Location and Construction
While it is usually described as in Washington, DC, most of the building is in Arlington, Virginia; the Arlington County Fire Department was the incident commander for the fire and rescue response after the 9-11 attacks.
Construction was directed by then-brigadier general Leslie Groves, who finished it under budget and ahead of schedule: finished in 16 months, at a cost of $83 million dollars (1942 value). That accomplishment led to his next assignment of managing the Manhattan Project, the U.S. program to build the first nuclear weapons.
At the time of construction, the dividing line between the District of Columbia and the Commonwealth of Virginia ran down the center of the Potomac River. Since part of the Pentagon rests on land dredged from the Potomac, some of which crossed the then-center of the river, it is true that part of the building is in Washington proper. It is an unusual and quite efficient building design. There are actually five concentric pentagonal buildings, each with five stories above the street level. The outermost and most prestigious is the "E Ring"; the innermost, which surrounds a reasonably pleasant courtyard, is the "A Ring". For many years, a snack bar in the center of the courtyard was called the Ground Zero Cafe. It was torn down for a new structure, but will rise again.
Radiating from the center are numbered "corridors". A given room is identified by floor, ring, corridor, and room number using a standard reference from the lower-numbered corridor. For example, the Office of the Secretary of Defense is in suite 3E880:
- Third floor
- E Ring
- 80th position from Corridor 8
The building has very few personnel elevators and stairways, with most movement between floors by broad ramps. This was partially done to save steel for WWII military requirements, but has proven to be quite convenient.
Contrary to urban legend, there are no deep, hardened shelters under the Pentagon. There are several full and partial floors below ground, but, for example, the National Military Command Center is on the second floor. Prior to the threat of vehicle-borne explosives, a commuter bus station was on ramps below the building; these have been closed and converted to some of the more pleasant, if underground, offices.
The Concourse, in the center of the building, contains a clinic, stores, and various amenities to reduce what would otherwise create a massive rush hour at the prime lunch time. Since the building does have many 24-hour offices, and long hours are common, the Concourse facilities provide some services at times that the usual commercial equivalents would not be open.
Its site remains a major transportation hub for the area; the Blue and Yellow lines of the Metrorail subway system meet in the transportation complex outside, and there are extensive lanes for commuter buses just outside the subway entrance. There is a heliport for executive transport helicopters, and even a shuttle boat, once scheduled but now by appointment only, from Bolling Air Force Base and other facilities on the Potomac River. Of the main entrances, the River Entrance is the most formal, and the Mall Entrance more heavily used.
General security features
Once a relatively public building, although always containing high-security areas, it remained accessible during the Vietnam War, with the largest of a series of demonstrations in October 1967. Immediately prior to that event, sponsored by the New Mobilization Committee to End the War in Vietnam, the Yippies counterculture group, led by Abbie Hoffman and Jerry Rubin, announced they would form a ring around the building by spiritual force. Causing the Yippies some confusion, the Pentagon security manager invited them into his office, offered coffee, and explained that he would be happy to cooperate, and indeed would enjoy seeing the event, but, since the building was so close to Washington National Airport, they would have to file a flight plan. They departed, in some confusion, when he offered to introduce them to the appropriate Federal Aviation Administration officials.
Not long before 9/11, the aging building was undergoing general renovation, and, as a general program of increasing physical security after the 1995 Oklahoma City bombing of the Alfred P. Murragh Federal Building in Oklahoma City. This included structural reinforcements and blast-resistant windows, with the renovations being done to one side of the Pentagon at a time. When American Airlines Flight 77 hit the building, it hit the area that was just finishing the renovation, and was not fully occupied.
Since 9/11, there is immensely more security on the grounds. In addition to conventional gates, power-operated barriers can be raised from the road surface to restrain a vehicle. On September 5, 2000, the Mall Entrance barrier accidentally raised underneath the limousine of the German Defense Minister, causing minor injuries. . Since a similar accident injured the Japanese defense minister two years earlier, there are persistent Washington rumors that the Italian defense ministry is not eager to visit, wondering if there might be remaining hard feelings about the Tripartite Pact. 
- Schmitt, Eric (March 22, 1992), "As It Cuts Back, Pentagon Faces Growing Pains", New York Times
- Kenneth H. Bacon, Assistant Secretary of Defense for Public Affairs (September 05, 2000), DoD News Briefing
- "Japanese defense minister injured in Pentagon accident", Cable News Network, September 21, 1998