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What Is the Pentagon? The U.S. Defense Headquarters Explained

What Is the Pentagon? The U.S. Defense Headquarters Explained
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“The Pentagon” means two things at once. It is a five-sided office building in Arlington, Virginia, across the Potomac River from Washington, D.C. It is also the everyday shorthand for the organisation that works inside it: the U.S. Department of Defense, which since September 2025 has also used the secondary name “Department of War”. This guide explains both. It covers how big the building really is, why it has five sides, who works there, how the department is organised, and where the name change legally stands in November 2026.

The Pentagon in numbers

  • Location: Arlington County, Virginia, United States.
  • Construction: ground broken on 11 September 1941 and completed on 15 January 1943, just 16 months later.
  • Office space: about 6.5 million square feet, three times the floor space of the Empire State Building.
  • Corridors: 17.5 miles (about 28 km).
  • Windows: 7,754.
  • Central courtyard: 5 acres.
  • Workforce: about 27,000 military and civilian personnel, according to Washington Headquarters Services (2025).
  • Walking time: about 7 minutes between the two furthest points.
  • Original cost: $63 million. The budget had been $35 million.

What is the Pentagon?

The Pentagon is the headquarters of the U.S. Department of Defense (DoD). The department’s own description calls it “the world’s largest low-rise office building”. The offices of the Secretary of Defense (now also styled Secretary of War), the Joint Chiefs of Staff and the leadership of the Army, Navy and Air Force all work under one roof. That is why “the Pentagon” has become a metonym, in the same way “the White House” stands for the presidency or “Downing Street” for the British government. A headline saying “the Pentagon announced” almost always means the Department of Defense, not the building.

Why does the Pentagon have five sides?

The shape is the result of an abandoned plan. As DoD’s own history explains, the first site chosen for the building was bordered on five sides by roads, so the architects designed a five-sided structure to fit. President Franklin D. Roosevelt then worried that a building on that spot would block the view of Washington from Arlington National Cemetery, so he moved the project south to its present location. The five-sided design stayed. The architectural style is described as “stripped classical”, and the building is listed on the U.S. National Register of Historic Places.

How the Pentagon was built in 16 months

In 1941 the War Department was growing fast and spread across many buildings in Washington and Virginia. Brigadier General Brehon B. Somervell, the Army’s construction chief, proposed housing the whole department under one roof. Congress approved the plan in the summer of 1941. Roosevelt signed the funding bill on 25 August 1941, and builders broke ground on 11 September 1941.

The construction effort was enormous. According to DoD:

  • More than 15,000 workers were on site around the clock.
  • Staff moved in before the building was finished.
  • Builders saved enough steel to build a battleship by using reinforced concrete.
  • The 689,000 tons of sand and gravel came from the Potomac River.

The officer who ran the project from August 1941, Colonel Leslie Groves, later directed the Manhattan Project.

The Pentagon and 9/11

Exactly 60 years after the groundbreaking, on 11 September 2001, American Airlines Flight 77 was flown into the building’s west side, killing 189 people. The plane hit a section of the building where a long-running renovation had just been completed. DoD notes that only about 800 of the roughly 4,500 people who would normally have worked there had moved back in. The new sprinkler system, structural reinforcement and blast-resistant windows also limited the damage. The renovation programme, which began in the 1990s, was completed in 2011 and added 70 passenger elevators to a building that had previously relied on ramps.

Who works in the Pentagon?

About 27,000 military and civilian employees work in the building. It is operated by Washington Headquarters Services (WHS), a DoD field activity. The main occupants are the top layers of the U.S. military establishment:

  • The Office of the Secretary of Defense (OSD), also styled the Office of the Secretary of War. This is the civilian leadership that sets policy and controls the budget.
  • The Joint Chiefs of Staff and the Joint Staff, led by the Chairman, who is the senior military adviser to the President.
  • The headquarters of the military departments: the Department of the Army, the Department of the Navy (Navy and Marine Corps) and the Department of the Air Force (Air Force and Space Force).

The combatant commands that actually run operations, such as CENTCOM, INDOPACOM or EUCOM, are not based in the Pentagon. They have their own headquarters around the world. The Pentagon is where policy, budgets, acquisition and force-generation decisions are made.

The Department of Defense: how it is organised

The Department of Defense was created after World War II by the National Security Act of 1947 and later amendments. It brought the old War Department and Navy Department under a single civilian secretary. According to the Congressional Research Service, DoD carries out its mission through five main sets of institutions:

  1. The Office of the Secretary of Defense.
  2. The Joint Chiefs of Staff and the Joint Staff.
  3. The military departments (Army, Navy, Air Force).
  4. The unified combatant commands.
  5. The defense agencies and field activities, such as the Defense Logistics Agency, the Missile Defense Agency and DARPA.

The department’s own budget documents and audits, including the long-running inability to pass a clean financial audit, are regularly examined by the U.S. Government Accountability Office (GAO). One recent example is our report on GAO’s finding that the Pentagon cannot account for 17,000 F-35 parts.

Department of Defense or Department of War?

On 5 September 2025, President Donald Trump signed Executive Order 14347, “Restoring the United States Department of War”. The order allows the department and its secretary to use “Department of War” and “Secretary of War” as secondary titles in official correspondence, public communications and ceremonial, non-statutory contexts. It does not rename the department in law. The order itself says that statutory references to the Department of Defense “shall remain controlling until changed subsequently by the law”.

The status in late 2026 is as follows:

  • The Congressional Research Service still describes the organisation as DoD “using a secondary Department of War designation” (CRS primer updated 31 July 2026).
  • In April 2026 the Pentagon formally asked Congress to codify the new name. It estimated the cost at about $52.5 million, according to Air Force Times.
  • The House passed its FY2027 National Defense Authorization Act, which contains rename language, by 216–212 on 22 July 2026. As of the latest CRS update, the Senate bill had not passed and no FY2027 NDAA had been enacted.

In practice, then, both names are in use. You will see “Department of War” and the war.gov domain on official websites and press releases, while laws, contracts and court documents still say “Department of Defense”. That is why this site, like most news organisations, refers to the department as “the Pentagon”, which is accurate under either name.

Can you visit the Pentagon?

Yes, but only on a pre-arranged guided tour. Tours are run by the Pentagon Tours office, must be booked in advance, and follow the eligibility and booking rules published on the official Pentagon Tours page. Visitors go through security screening and are escorted throughout. The outdoor National 9/11 Pentagon Memorial next to the building is open to the public without a tour.

Frequently asked questions

Why is it called the Pentagon?

Because the building is a pentagon, a five-sided shape. It was designed for a site bordered by five roads and kept that shape after the site moved.

Is the Pentagon the same as the Department of Defense?

Strictly speaking, the Pentagon is the building that serves as DoD headquarters. In everyday use, and in news reporting, “the Pentagon” means the Department of Defense itself.

How many people work at the Pentagon?

About 27,000 military and civilian personnel, according to Washington Headquarters Services.

Has the Department of Defense been renamed the Department of War?

Not in law. Since 5 September 2025 it may use “Department of War” as a secondary title under Executive Order 14347. A permanent change requires an act of Congress, which had not been enacted as of the latest CRS update.

Is the Pentagon the biggest office building in the world?

DoD calls it the world’s largest low-rise office building. It has about 6.5 million square feet of office space and 17.5 miles of corridors.

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