Even today, 75 years later, the numbers are staggering: The massive building, constructed to bring the U.S. Department of War under a single roof in the lead-up to World War II, sprawls over 29 acres, an area larger than the footprint of the Great Pyramid of Cheops, and is still the largest of any office building in the world, capable of containing five U.S. Capitol Buildings. The Pentagon sits atop 41,492 piles driven into the bottomlands of the Potomac River, from which 680,000 tons of sand and gravel were dredged to make concrete for the project. The building’s interior contains 6.5 million square feet, about half of which is used as office space. Its lawns total 200 acres and its 67-acre parking lot can accommodate 8,770 vehicles. The total length of its corridors is more than 17 miles. 23,000 military and civilian employees of the Department of Defense work there, along with about 3,000 support personnel – a population roughly equal to that of New London, Connecticut. Six ZIP codes are assigned to it.

It was designed and built with great speed and pragmatism, about 17 months from conception to dedication – slightly longer than it takes some contractors today to finish a new house. In spite of – or maybe because of – its designers’ prosaic avoidance of embellishment, the Pentagon hasn’t suffered the ignominy directed at other hastily conceived government buildings. To the rest of the world, it’s more than a building: It’s a symbol of immovable might, of the dominance of the U.S. military. “The Pentagon” is today a metonym, a name that stands in for the entire military and all of its service branches.

In July 1941, just after Nazi Germany had expanded its aggression beyond Europe and invaded the Soviet Union, the U.S. War Department was a bureaucracy of 24,000 people scattered among 23 buildings in and around Washington, D.C. The War Department was in the midst of an unprecedented peacetime mobilization; the draft bill, approved months earlier, had already increased the ranks of the Army to about a million-and-a-half soldiers, and it was estimated that the number of people needed to administer and support this Army would swell to 30,000 by the end of the year.

The government had already approved and begun construction of a new War Department headquarters, at 21st and C Streets in Washington’s Foggy Bottom neighborhood, but the War Department already had outgrown it even before it was completed. It was briefly known as the “War Department Building,” but never became department headquarters. A few years later, the State Department moved in, and today still occupies the 1.4 million-square-foot complex now known as the Harry S. Truman Building.

Secretary of War Henry L. Stimson knew the man in charge of accommodating the growing Army – building the dozens of camps where new soldiers would be trained and housed, as well as the new administrative spaces – was the right person for the job. The chief of the Army’s Construction Division, then a small bureau within the Quartermaster Corps, was Gen. Brehon B. Somervell. He’d come to Stimson’s attention during his leadership of the Depression-era Works Progress Administration in New York City. Somervell, contending with hot-tempered Mayor Fiorello LaGuardia, the city’s 200,000 workers, and the intractable labor unions that represented them, had brought them all to heel and built the $45 million, 558-acre New York Municipal Airport (later renamed LaGuardia Airport) in less than two years.