A window display of the 1940s at the Pentagon Building History exhibit includes an original Pentagon clock, one of thousands that hung in the building. A 48-star American flag stands in the corner, near a wall-sized map showing the world circa 1940 and various patriotic posters. (Bill Hopper/Washington Headquarters Service)

In the summer of 2009, as the multiyear, multibillion-dollar renovation of the Pentagon rushed toward its conclusion, building supervisors confronted a sobering thought: By 2011, almost nothing would remain of the historic military headquarter’s original interior.

Nearly every square inch of the distinctive, World War II-era decor would be gutted and converted into ubiquitous, modern Washington office space.

“It was almost a little bit of remorse — is there something we can do to preserve a piece of this?” recalled Bill Brazis, director of Washington Headquarters Services, the Defense Department agency that manages the building. “It was the idea that we were losing a bit of history if we didn’t pause.”

They did pause. After raising the question up the chain of command and getting a nod of approval from then-Secretary of Defense Robert M. Gates, building managers agreed to spare a small swath of territory on the third floor, along Corridor 2 off the C ring.

The result is the “Pentagon Building History Exhibit,” which officially opened this month. Two offices, preserved and restored to earlier times, sit behind a plate-glass window along the corridor. Visitors can walk through a third display room stripped to its concrete slab and terra-cotta walls and stroll down a tiny stretch of original hallway, the only unrenovated segment left from the Pentagon’s 17 miles of corridors.

At 1,600 square feet, the exhibit is modest for a 6.5 million-square-foot building. At that late stage in the 17-year renovation, it was all that the planners dared take in a headquarters where military services and defense agencies jealously protect their territory.

“We had to strike a balance,” Brazis said. “We didn’t want to take operating space from people who need it. It was not in the cards to preserve a whole corridor or ring.”

The first room recreates the Pentagon of World War II, soon after it opened in 1942. The walls are a sickly green familiar to generations of employees; curators painstakingly matched the paint to the original color. Along the back wall, original windows overlook the concrete walls of a light well.

The room is populated by life-sized photo cutouts of Pentagon workers taken from historical photographs. “It was a tossup between mannequins and photos,” said Albert Jones, curator of historical exhibits for the Office of the Secretary of Defense, who oversaw the decor. “People have liked these. They look more authentic.”

At one desk, a secretary perched in her chair in front of a Royal typewriter and rotary telephone cheerily holds up a memo from the Army adjutant general’s office. Another secretary files papers, while a male colleague in a double-breasted suit chats on the phone. One fellow clasps a cigarette; stashed in his desk are bottles of whiskey and brandy — authentic empties that renovation workers found behind Pentagon walls.

The room includes an original Pentagon clock, one of thousands that hung in the building. A 48-star American flag stands in the corner, near a wall-sized map showing the world circa 1940 and various patriotic posters.

Not everything is authentic. The floor, for example, does not include the original asbestos tiles, removed as a health risk. Many of the artifacts, including desks and chairs, are not original to the Pentagon and were collected by a contractor. Countless thousands of original artifacts were disposed of during renovation.

A second room, also with photographic cutouts, recreates a typical office before the start of renovation in 1994, when the Pentagon was at its height of decay. The building had not met safety codes in 40 years and was nearly crippled by a water main break shortly after the Iraqi invasion of Kuwait in 1990.

This second room includes little touches that only a Pentagon veteran could appreciate. The vents on the original Weathermaster heating and cooling unit are covered with cheesecloth, a custom of workers to defend against black soot blowing out.

A portable space heater — strictly against the rules but standard equipment in many chilly offices — sits at the foot of another desk. A workman on a ladder reaches through the acoustic tile ceiling, perhaps fixing a pipe dripping unknown liquids on office workers.

The desks are crowded with bulky Zenith and Heath computers, complete with floppy disks and reams of operating manuals.

The original C Ring hallway features heavy green doors with smoked-glass windows and an old ceramic water fountain that doesn’t work, another authentic touch. Open doors reveal old electrical and telecommunications closets with spaghetti-like jumbles of wires. “That’s just the way we found it,” Brazis said.

In the display room, multimedia images flash on the concrete, showing the race to build the Pentagon during World War II as well as the Phoenix Project, the effort to repair the building after the terrorist attack of Sept. 11, 2001.

A rubber rat, a reminder of the days when large rodents stalking the corridors struck fear in the hearts of Pentagon workers, sits by a wall.

The exhibit will be incorporated into many of the regular tours that take visitors through the building. Along with the original walnut staircase and marble steps at the Mall entrance, the original tiles on the buildings ramps and a few other touches, the space is all that’s left of the Pentagon’s interior.

“We all sometimes feel like we’re in an office building like any other office building, but it’s not just an office building,” Brazis said. “Hopefully this will be a place for people to reflect a little bit on the history of this building.”