Before the Pentagon was built, the
United States Department of War was headquartered in the
Munitions Building, a temporary structure erected during
World War I along
Constitution Avenue on the
National Mall. The War Department, which was a civilian agency created to administer the
U.S. Army, was spread out in additional temporary buildings on the National Mall, as well as dozens of other buildings in Washington, D.C.,
Virginia. In the late 1930s a
new War Department Building was constructed at 21st and C Streets in
Foggy Bottom but, upon completion, the new building did not solve the department's space problem and ended up being used by the
Department of State.
World War II broke out in Europe, the War Department rapidly expanded in anticipation that the United States would be drawn into the conflict. Secretary of War
Henry L. Stimson found the situation unacceptable, with the Munitions Building overcrowded and the department spread out.
Stimson told U.S. President
Franklin D. Roosevelt in May 1941 that the War Department needed additional space. On July 17, 1941, a congressional hearing took place, organized by Virginia congressman
Clifton Woodrum, regarding proposals for new War Department buildings. Woodrum pressed Brigadier General
Eugene Reybold, who was representing the War Department at the hearing, for an "overall solution" to the department's "space problem" rather than building yet more temporary buildings. Reybold agreed to report back to the congressman within five days. The War Department called upon its construction chief, General
Brehon Somervell, to come up with a plan.
Main Navy Building (foreground) and the
were temporary structures built during
World War I
. The Munitions Building served as the Department of War headquarters for several years before moving into the Pentagon.
Government officials agreed that the War Department building, officially designated Federal Office Building No 1, should be constructed across the Potomac River, in
Arlington County, Virginia. Requirements for the new building were that it be no more than four stories tall, and that it use a minimal amount of steel. The requirements meant that, instead of rising vertically, the building would be sprawling over a large area. Possible sites for the building included the Department of Agriculture's Arlington Experimental Farm, adjacent to
Arlington National Cemetery, and the obsolete
Hoover Field site.
The site originally chosen was
Arlington Farms which had a roughly pentagonal shape, so the building was planned accordingly as an irregular pentagon.
 Concerned that the new building could obstruct the view of Washington, D.C., from Arlington Cemetery, President Roosevelt ended up selecting the Hoover Airport site instead.
 The building retained its pentagonal layout because a major redesign at that stage would have been costly, and Roosevelt liked the design. Freed of the constraints of the asymmetric Arlington Farms site, it was modified into a regular
pentagon which resembled the
star forts of the
On July 28
Congress authorized funding for a new Department of War building in Arlington, which would house the entire department under one roof,
 and President Roosevelt officially approved of the Hoover Airport site on September 2.
 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.
 In addition to the Hoover Airport site and other government-owned land, construction of the Pentagon required an additional 287 acres (1.16 km2), which were acquired at a cost of $2.2 million.
 The Hell's Bottom neighborhood, a slum with numerous pawnshops, factories, approximately 150 homes, and other buildings around
Columbia Pike, was also cleared to make way for the Pentagon.
 Later 300 acres (1.2 km2) of land were transferred to Arlington National Cemetery and to
Fort Myer, leaving 280 acres (1.1 km2) for the Pentagon.
Contracts totaling $31,100,000 were finalized with McShain and the other contractors on September 11, and ground was broken for the Pentagon the same day.
 Among the design requirements, Somervell required the structural design to accommodate floor loads of up to 150 pounds per square foot, which was done in case the building became a records storage facility at some time after the end of the current war.
 A minimal amount of steel was used as it was in short supply during World War II. Instead, the Pentagon was built as a reinforced concrete structure, using 680,000 tons of sand dredged from the
Potomac River, and a lagoon was created beneath the Pentagon's river entrance.
 To minimize steel usage, concrete ramps were built rather than installing elevators.
Indiana limestone was used for the building's façade.
Northwest exposure of the Pentagon's construction underway, July 1, 1942.
Architectural and structural design work for the Pentagon proceeded simultaneously with construction, with initial drawings provided in early October 1941, and most of the design work completed by June 1, 1942. At times the construction work got ahead of the design, with different materials used than specified in the plans. Pressure to speed up design and construction intensified after the
attack on Pearl Harbor on December 7, 1941, with Somervell demanding that 1,000,000 sq ft (9.3 ha) of space at the Pentagon be available for occupation by April 1, 1943.
 David J. Witmer replaced Bergstrom as chief architect on April 11 after Bergstorm resigned due to charges, unrelated to the Pentagon project, of improper conduct while he was president of the
American Institute of Architects.
 Construction was completed January 15, 1943.
The construction of the Pentagon was done during a time when parts of the U.S. were under legally-mandated
racial segregation. This had structural consequences to the design of the building. Under the supervision of colonel
Leslie Groves, the decision to have separate eating and lavatory accommodations for white persons and black persons was made and carried out. The dining areas for black persons were put in the basement and on each floor there were double toilet facilities separated by gender and race. These measures of segregation were said to have been done in compliance with the U.S. Commonwealth of Virginia's racial laws. The Pentagon as a result has twice the number of toilet facilities needed for a building of its size.
U.S. President Franklin Roosevelt issued Executive Order 8802 on June 25, 1941, to end discrimination in the national defense industry on the basis of race, creed, color, or national origin. When the President visited the Pentagon before its dedication, he questioned Groves regarding the number of washrooms and ordered him to remove the 'Whites Only' signs. Until 1965 the Pentagon was the only building in Virginia where segregation laws were not enforced.
The soil conditions of the Pentagon site, located on the Potomac River floodplain, presented challenges to engineers, as did the varying elevations across the site, which ranged from 10 to 40 feet (3.0–12.2 m) above sea level. Two retaining walls were built to compensate for the elevation variations, and
cast-in-place (Franki) piles were used to deal with the soil conditions.
 Construction of the Pentagon was completed in approximately 16 months at a total cost of $83 million. The building is 77 feet (23 m) tall, and each of the five sides of the building is 921 feet (281 m) long.
Because of the pressing needs of the war, people started working in the Pentagon before it was completed. The Pentagon was built one wing at a time, and after the first wing was finished, employees started to move into that wing while construction was continuing on the other wings.
The Pentagon became a focal point for protests against the
Vietnam War during the late 1960s. A group of 2,500 women, organized by
Women Strike for Peace, demonstrated outside of Secretary of Defense
Robert S. McNamara's office at the Pentagon on February 15, 1967.
 In May 1967, a group of 20 demonstrators held a sit-in outside the Joint Chiefs of Staff's office, which lasted four days before they were arrested.
 In one of the better known incidents, on October 21, 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"), where they were confronted by some 2,500 armed soldiers. During the protest, a famous picture was taken, where
George Harris placed
carnations into the soldiers' gun barrels.
 The march concluded with an attempt to "exorcise" the building.
On May 19, 1972, the American radicals known as the
Weather Underground Organization successfully planted and detonated a bomb in a fourth-floor women's restroom in the Pentagon. They announced it was in retaliation for the
Nixon administration's bombing attacks on
Hanoi during the final stages of the Vietnam War.
On March 17, 2007, 4,000 to 15,000 people (estimates vary significantly) protested against the
 The protesters marched from the
Lincoln Memorial, down Washington Boulevard to the Pentagon’s north parking lot.
From 1998 to 2011, the Pentagon underwent a major renovation, known as the Pentagon Renovation Program. This program, completed in June 2011, involved the complete gutting and reconstruction of the entire building in phases to bring the building up to modern standards, removing
asbestos, improving security, providing greater efficiency for Pentagon tenants, and sealing of all office windows.
As originally built, most Pentagon office space consisted of open bays which spanned an entire ring. These offices used
cross-ventilation from operable windows instead of air conditioning for cooling. Gradually, bays were subdivided into private offices with many using
window air conditioning units. With renovations now complete, the new space includes a return to open office bays, a new Universal Space Plan of standardized office furniture and partitions developed by Studios Architecture.
September 11 attacks
9/11 anniversary illumination
The Pentagon in April 2002.
The Pentagon, minutes after American Airlines Flight 77 crashed into it
A fire at the Pentagon, with police and EMS in the foreground
On September 11, 2001, the 60th anniversary of the Pentagon's groundbreaking, a team of 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. All 59 civilians and the 5 terrorists on the airliner were killed, as were 70 civilians and 55 military personnel who were in the building. The impact of the plane severely damaged the outer ring of one wing of the building and caused its partial collapse.
 At the time of the attacks, the Pentagon was under renovation and many offices were unoccupied, resulting in fewer casualties. Only 800 of 4,500 people who would have been in the area were there because of the work. Furthermore, the area hit, on the side of the Heliport facade, was the section best prepared for such an attack. The renovation there, improvements which resulted from the
Oklahoma City bombing, had nearly been completed.
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 thick and 2,500 pounds each—that stayed intact during the crash and fire. It had fire doors that opened automatically and newly built exits that allowed people to get out.
Contractors already involved with the renovation were given the added task of rebuilding the sections damaged in the attacks. This additional project was named the "
Phoenix Project," and was charged with having the outermost offices of the damaged section occupied by September 11, 2002.
When the damaged section of the Pentagon was repaired, a small indoor memorial and chapel were included, located at the point of impact. For the fifth anniversary of the September 11, 2001 attacks, a memorial of 184 beams of light shone up from the center courtyard of the Pentagon, one light for each victim of the attack. In addition, an American flag is hung each year on the side of the Pentagon damaged in the attacks, and the side of the building is illuminated at night with blue lights. After the attacks, plans were developed for an outdoor memorial, with construction underway in 2006. This
Pentagon Memorial consists of a park on 2 acres (8,100 m2) of land, containing 184 benches, one dedicated to each victim. The benches are aligned along the line of Flight 77 according to the victims' ages, from 3 to 71. The park opened to the public on September 11, 2008.
On March 4, 2010, at 6:40 pm, two police officers working for the
Pentagon Force Protection Agency were shot near an entrance to the Pentagon and fired back with their pistols at the suspect. The officers were injured but were treated in a hospital and released. The suspect, identified as
John Patrick Bedell (age 36), died at the hospital. No clear motive was established.
 On October 19, 2010, shortly before 5 am, an unidentified gunman shot at the south side of the building, shattering windows on the third and fourth floors.
On August 23, 2011, a 5.8 magnitude earthquake in
Mineral, Virginia, shook the Pentagon.
 The building suffered minor damage, with flooding from broken pipes.