On September 11, 1941, construction began on the Pentagon. Exactly 60 years later, on September 11, 2001, terrorists targeted the building in one of the deadliest attacks on American soil.

The Pentagon: Born in the Shadow of War

On 11 September 1941, ground was broken on what would become the world’s largest low-rise office building. Exactly 60 years later, on 11 September 2001, 184 people were killed when hijacked American Airlines Flight 77 struck the complex. Designed out of wartime desperation, the Pentagon was never intended to be an icon—it was a rapid, practical answer to a severe logistical crisis.

Conception & Emergency Design
By mid-1941, the US War Department’s 24,000 local personnel were scattered across 17 buildings. Anticipating total war, Brigadier General Brehon B. Somervell, head of the Construction Division, demanded a unified headquarters housing 40,000 people.

Over a single weekend in July 1941, Lieutenant Colonel Hugh J. Casey and architect George E. Bergstrom drafted the five-sided design. The shape was dictated by the initial site—Arlington Farms—which was constrained by five intersecting roads. President Franklin D. Roosevelt later moved the site to protect the sightlines of Arlington National Cemetery, but the pentagonal plan remained.

Construction Footprint & Logistics
Work moved around the clock under giant arc lights. Following the 7 December 1941 attack on Pearl Harbor, the project’s scale was expanded by over 50%. Overseen by Colonel Leslie R. Groves—who would later direct the Manhattan Project—the build prioritized speed and resource conservation. To save structural steel for tanks and ships, builders used 435,000 cubic yards of reinforced concrete, mixing sand and gravel dredged directly from the nearby Potomac River.

Metric	Detail
Total Floor Area	~6.5 million sq. ft.
Footprint Materials	435,000 yd³ concrete, 43,000 tons steel, 680,000 tons aggregate
Peak Workforce	~15,000 tradesmen and 1,000 architects
Layout	5 concentric rings (A–E), 10 radial corridors (17.5 miles total)
Completion Time	16 months (11 Sept 1941 – 15 Jan 1943)
Final Cost	$63 million (vs. initial $35 million estimate)

Immediate Occupation & Legacy
Employees moved in long before completion. Section A opened on 30 April 1942 for 300 Ordnance Department workers; by December, 22,000 personnel worked alongside ongoing construction.

Initially mocked as “Somervell’s Folly” and expected to be decommissioned post-war, the building became permanent as the US transitioned into a global superpower during the Cold War. Today, it remains the central nerve center of American military power.

The Pentagon eventually provided workspace for approximately 33,000 people, making it one of the largest office buildings ever constructed.

Why five sides?

The Pentagon’s unusual shape has generated countless stories over the years. Its five sides were not originally intended as some symbolic representation of American military power.

The pentagonal design largely resulted from the practical requirements of the site and the need to create an efficient headquarters. An earlier proposal was based on a four-sided design, but the site’s boundaries and surrounding roads encouraged the five-sided solution.

The shape proved remarkably effective. The concentric rings and radial corridors meant that people could move around the building with unusual efficiency. At the time, the designers estimated that it was possible to walk from one point in the building to another in only a few minutes.

The building’s five sides eventually became so closely associated with the American military that the Pentagon itself became shorthand for the Department of Defense.

A headquarters for a global superpower

When the Pentagon opened, the United States was fully engaged in the Second World War. The building therefore became the administrative centre of an enormous military machine fighting on multiple continents.

The original War Department was reorganized during and after the war. In 1947, the National Military Establishment was created, and in 1949 it was renamed the Department of Defense. The Pentagon became the department’s headquarters.

During the Cold War, the building became one of the principal centres from which American military policy was planned and coordinated. Presidents, generals, diplomats and defence officials passed through its corridors as the United States confronted the Soviet Union and managed conflicts around the world.

More than a building

The Pentagon is an unusual monument to the circumstances of its creation. It was not designed as a monument, nor was it intended to become an architectural symbol. It was built because the United States urgently needed somewhere to coordinate a rapidly expanding military establishment.

Its construction demonstrated the extraordinary industrial and organizational capacity of the United States on the eve of its entry into the Second World War. Within months, a vast headquarters had risen from the ground in Arlington.

Today, the Pentagon remains one of the world’s most recognizable buildings. Its five sides, enormous scale and military significance have made it an enduring symbol of American defence power.

Yet perhaps the most striking fact about the Pentagon is not its size or its distinctive shape. It is the speed with which it was built. Ground was broken on 11 September 1941, just three months before the United States entered the Second World War, and within little more than a year the world’s largest military headquarters was already taking shape.

The Pentagon was, quite literally, built for a nation preparing for war—and its construction became an early demonstration of the vast mobilization that would help transform the United States into a global military power.

And on 11 September 1941, as workers began laying the foundations, nobody could have known that the building rising before them would still be standing sixty years later—and that the same date would one day be permanently associated with one of the darkest days in its history.

Sources:

https://history.defense.gov/DOD-DOW-History/Pentagon/History

https://en.wikipedia.org/wiki/The_Pentagon

https://www.britannica.com/topic/Pentagon

https://www.war.gov/News/Feature-Stories/story/Article/1867440/pentagon-history-7-big-things-to-know