The Pentagon is the headquarters building of the United States Department of Defense, in Arlington County, Virginia, across the Potomac River from Washington, D.C.

Five Al-Qaeda hijackers flew American Airlines Flight 77 into the western side of the building, killing themselves and 184 other people, including 59 on the airplane and 125 in the Pentagon.

[9] Following the attacks, the western side of the building was repaired, with a small indoor memorial and chapel added at the point of impact.

[citation needed] On the north side of the building, the Mall Entrance, which also features a portico, leads out to a 600 ft-long (180 m) terrace that is used for ceremonies.

The River Entrance, which features a portico projecting out twenty ft (6 m), is on the northeast side, overlooking the lagoon and facing Washington.

A stepped terrace on the River Entrance leads down to the lagoon; and a landing dock was used until the late 1960s to ferry personnel between Bolling Air Force Base and the Pentagon.

The south parking lot adjoins the southwest façade, and the west side of the Pentagon faces Washington Boulevard.

Secretary of War Henry L. Stimson found the situation unacceptable, with the Munitions Building overcrowded and department offices spread out in additional sites.

On 17 July 1941, a congressional hearing took place, organized by Representative Clifton Woodrum (D-VA), regarding proposals for new War Department buildings.

Requirements for the new building were that it be no more than four stories tall, and that it use a minimal amount of steel to reserve that resource for war needs.

[22] Concerned that the new building could obstruct the view of Washington, D.C., from Arlington Cemetery, President Roosevelt selected the Hoover Airport site instead.

[26] While the project went through the approval process in late July 1941, Somervell selected the contractors, including John McShain, Inc. of Philadelphia, which had built Washington National Airport in Arlington, the Jefferson Memorial in Washington, and the National Naval Medical Center in Bethesda, Maryland, along with Wise Contracting Company, Inc. and Doyle and Russell, both from Virginia.

[28] The Hell's Bottom neighborhood, consisting of numerous pawnshops, factories, approximately 150 homes, and other buildings around Columbia Pike, was cleared to make way for the Pentagon.

[28] Land for parking and roads serving the complex was acquired by the exercise of eminent domain in February 1942 over the African-American neighborhood of Queen City, in East Arlington, displacing hundred of Black families, who were given four to six weeks to leave their homes.

[30][31] Contracts totaling $31,100,000 (equivalent to $497 million in 2023[1]) were finalized with McShain and the other contractors on 11 September 1941, and ground was broken for the Pentagon the same day.

Pressure to speed up design and construction intensified after the attack on Pearl Harbor on 7 December 1941, with Somervell demanding that 1 million sq ft (9.3 ha) of space at the Pentagon be available for occupation by 1 April 1943.

[42][43] The Pentagon was designed in accordance with the racial segregation laws in force in the state of Virginia at the time, with separate eating and lavatory accommodations for white and black persons.

Judge William Hastie, the Black civilian aide to Secretary of War Stimson, soon learned of the incident and was able to get an investigation authorized.

Upon hearing about this general Brehon B. Somervell ordered for there to be "discontinuance of any enforced segregation of negro employees in the cafeterias in the Pentagon building.

[45] On the building's main concourse is the Hall of Heroes, opened 1968[47] and dedicated to the more than 3,460 recipients of the Medal of Honor,[note 1] the United States' highest military decoration.

[63] In one of the better known incidents, on 21 October 1967, some 35,000 anti-war protesters organized by the National Mobilization Committee to End the War in Vietnam, gathered for a demonstration at the Defense Department (the "March on the Pentagon").

[68] On September 11, 2001, coincidentally the 60th anniversary of the Pentagon's start of construction, five al-Qaeda affiliated hijackers took control of American Airlines Flight 77, en route from Washington Dulles International Airport to Los Angeles International Airport, and deliberately crashed the Boeing 757 airliner into the western side of the Pentagon at 9:37 am EDT as part of the September 11 attacks.

The impact of the plane severely damaged the outer ring of one wing of the building and caused its partial collapse.

[70][72] It was the only area of the Pentagon with a sprinkler system, and it had been reconstructed with a web of steel columns and bars to withstand bomb blasts.

The steel reinforcement, bolted together to form a continuous structure through all of the Pentagon's five floors, kept that section of the building from collapsing for 30 minutes—enough time for hundreds of people to crawl out to safety.

The area struck by the plane also had blast-resistant windows—2 inches (5 cm) thick and 2,500 pounds (1,100 kg) each—that stayed intact during the crash and fire.

[74][75][76] When the damaged section of the Pentagon was repaired, a small indoor memorial and chapel were added at the point of impact.

The misinformation was based on a fabricated image of an explosion near the Pentagon, which was quickly spread on social media and picked up by several platforms.