ON A WARM AND RAINY THURSDAY EVENING IN JULY 1941, inside a War Department office in Washington, a small group of Army officers hastily assembled for a meeting and listened in disbelief to the secret plan outlined by their commander.

The general spoke in the velvety Southern accent of his native Arkansas. He was not in uniform -- Army policy kept officers in civilian clothes so as to disguise from Congress the burgeoning military population in Washington -- but he cut an immaculate figure, with his trim build, combed-back, graying hair and neatly groomed mustache. Over the past eight months, the officers of the Army's Construction Division had grown accustomed to bold and quick action from their chief. At age 49, Brig. Gen. Brehon Burke Somer-vell had earned a reputation as a smooth but ruthless operator. "Dynamite in a Tiffany box" was how an associate later described him. Now Somervell turned his eyes -- "the keenest, shrewdest, most piercing eyes one is likely to meet," in the words of one observer -- toward his chief of design, Lt. Col. Hugh "Pat" J. Casey.

The War Department needed a new headquarters, Somervell said. The building he wanted to create was too big to fit in Washington and would have to go across the Potomac River in Arlington. It would be far larger than all the great structures of the city, including the U.S. Capitol. Somervell wanted a headquarters big enough to hold 40,000 people, with parking for 10,000 cars. It would contain 4 million square feet of office space -- almost twice as much as the Empire State Building. Yet it must be no more than four stories high -- a tall building would obstruct views of Washington and require too much steel, urgently needed for battleships and weapons.

The War Department would occupy the new headquarters within half a year, Somervell instructed. "We want 500,000 square feet ready in six months, and the whole thing ready in a year," the general said. Somervell ended the meeting with orders to have the basic design plans for the building by Monday morning.

Washington was consumed by war anxiety. Three weeks earlier, Adolf Hitler, already in control of much of Europe, had launched a surprise attack on the Soviet Union. President Franklin D. Roose-velt, alarmed by Nazi gains, had declared a national emergency on May 27. The War Department in Washington was growing at an explosive rate, its 24,000 workers spread in 17 buildings, including apartment buildings, private homes and several rented garages. Gen. George C. Marshall, the Army chief of staff, needed a quick solution and turned to Somervell to construct temporary buildings for the headquarters. At a congressional hearing July 17, Rep. Clifton A. Woodrum, a powerful Virginia congressman, signaled interest in finding an "overall solution" to the War Department's problem. Somervell took that as a signal for a permanent fix, and the Pentagon, as it would become known, was launched that evening.

The first problem was where to put it -- "incidentally, the largest office building in the world," Casey later noted dryly. Energetic and experienced, Casey was one of the Army's most brilliant engineers, and he quickly saw big problems with the location Somervell had chosen. Washington-Hoover Airport, at the foot of the 14th Street Bridge in Arlington, had just been replaced with a modern airfield, National Airport, about a mile downriver. Somervell -- eager to win the Virginian's blessing for the project -- had seized upon the old airport site, but the low-lying land, which was subject to flooding, worried Casey.

When Casey asked Somervell whether other sites near the airport might be used, the general did not rule it out. Scanning a map, Casey's practiced eye quickly zeroed in on a 67-acre tract about a half-mile upriver from Washington-Hoover. It was Arlington Farm, just east of Arlington National Cemetery. Like the adjacent cemetery, the land had been part of the grand estate of Robert E. Lee that had been confiscated by Union troops in the spring of 1861 for the defense of Washington. In 1900, Congress transferred 400 acres of the Arlington estate to the Department of Agriculture to use as an experimental farm. In September 1940, Roosevelt approved the return of Arlington Farm to the War Department for use by infantry and cavalry troops at neighboring Fort Myer. Perched on a hill above the Potomac, just below the Lee mansion and overlooking Memorial Bridge, Arlington Farm was one of the most prominent sites in the Washington area.

Late on Friday afternoon, July 18, George Edwin Bergstrom got to work. A formal man with a brusque manner, his dark hair whitening at the temples, Bergstrom was an accomplished and experienced architect, now in charge of the largest project of his long career. He gathered with his assistants at the division headquarters.

Bergstrom led the deliberations. The restrictions were confounding, given the space they needed. The easiest solution, constructing a tall building, was out. They would have to spread out horizontally. But how? A square building that size -- with the enormous interior distances to be covered -- was too unwieldy, as was a rectangle. The Arlington Farm tract had a peculiar asymmetrical pentagon shape bound on five sides by roads or other divisions. Finally, guided by the odd shape of the plot, they designed an irregular pentagon. A sketch by Socrates Thomas Stathes, a young War Department draftsman, showed a square with a corner cut off, more or less matching the tract's shape. It was really two buildings, a five-sided ring surrounding a smaller one of the same shape.

All through the weekend, the architects refined the design. The interior of the outer ring was lined with 49 barracks-like wings, sticking in like the teeth of a comb. The smaller ring had 34 exterior wings, all pointing toward the outer ring. The wings were 50 feet wide and 160 feet long, separated from each other by 30-foot-wide open-air "light courts." Corridors connected the two rings on the ground and third floors. Only the most senior officials would have private offices. Allowing 100 square feet per worker, the building could hold 40,000 employees.

There were many problems with the irregular design. The pattern was awkward, and the routes between wings of the two buildings were circuitous. Lacking symmetry, with rows of wings sticking out, the building was frankly quite ugly. Yet, given the site, the pentagonal design had one overriding virtue, Stathes remembered more than 60 years later: "It fit."

The whole idea seemed nonsensical to Secretary of War Henry L. Stimson. The War Department had just opened a new building the previous month in Foggy Bottom, but it had quickly proven inadequate and too small. How could the War Department propose to build a new headquarters so soon?