The Pentagon, 40 years old today, is more of an image than a fact, more an opinion than a building.

This seems strange--the building is one of the largest man-made things in the world. But it's true. The place tends to work more on our minds than on our senses.

And for understandable reasons: the buzz of the bureaucracy behind those near-1,000-foot-long limestone walls has to do with doom and salvation, themes likely to captivate everybody's imagination and even to alter the vision so that the walls become larger by far than life. Depending upon a person's political point of view, or degree and kind of paranoia, this distortion can be good news or bad, making the place seem more fortress-like and protective, or more threatening and awesomely impersonal.

Actually, in architectural terms, the Pentagon is an exercise of the opposite kind, a huge structure that in many ways belies its real size and its immense symbolic importance. Amazingly, it nearly comes across as what architects call a "background" building. Not to call it a masterpiece, or anything like, it is nonetheless a significant achievement, the more so for the trying circumstances under which it was created.

A sign in a tourist's hallway in the Pentagon proclaims: "PROBLEM: Spring 1941. A period of increasing international tension. The War Department cannot function efficiently with its 24,000 people scattered through the Washington area. SOLUTION: House them in a single building. THE PENTAGON."

Amazingly, this terse description is not too far from actual fact. In basic concept the Pentagon was designed under orders during a single weekend in the summer of 1941, by a small group of Army engineers, construction experts, and a dollar-a-year man named George Edwin Bergstrom, an architect from Los Angeles (and president of the American Institute of Architects) who, upon request, contributed his services to the War Department.

Many people, including President Franklin D. Roosevelt, at first objected to its size and its site. In fact FDR, responding to criticisms from the Washington review agencies (one of them headed by his uncle, Frederic Delano) forced the Army to change its plans somewhat, moving the site three-quarters of a mile south of the original proposal (closer to Memorial Bridge) and, hard to imagine, reducing the project in size.

The change in site, strenuously objected to by the Army because of the terrible drainage in the new location (it had to be built upon concrete pylons, which still are being added today under sinking portions), was of tremendous long-term benefit to Washington, for it preserved the great vista across Memorial Bridge and made the Pentagon more of an island unto itself.

Paradoxically, this also meant the Pentagon would end up being much less obtrusive: surrounded by highways, sitting low on a plain beneath the high Arlington ridge, the Pentagon interferes hardly at all with any important views, from either side of the river. When you come over the ridge on Columbia Pike, with the the monumental city spreading across the horizon, the Pentagon is the very last thing you see.

Up close, the building seems much less awesome than it should, given its actual size. It is in outline a respectful, traditional Washington building: low, divided into the traditional base, middle and top, symmetrical, punctuated by projecting colonnaded pavilions on each facade to help break plane and mass--in other words, in its limited way a crisp piece of work. The style of stripped classicism, the Esperanto for public buildings at the time (and Roosevelt's preferred Washington mode), may never have been so gainfully employed as on those long Pentagon walls.

The Pentagon, in short, was a no-nonsense building. Its construction schedule of 16 months, no matter what the price (which ended up to be about $83 million) was a miracle. Its size is what architectural critics picked up on back then. (They hated the style or lukewarmly endorsed it.) Architectural Forum, for instance, enthused: "A building so enormous takes on an entirely new quality of interest and excitement, a quality which depends not on the 'architecture' but on its size and the problems that go with it."

Spurred by the necessities of war, the government was building the kind of thoroughly planned environment modern architects had so far been able only to write and dream about, and architects and planners worldwide were impressed. Much has been said about the Pentagon's labyrinthine qualities, but in fact the building is extremely rational, and extremely modernist, in plan. As a sort of comprehensive pedestrian city in miniature, it was extremely advanced for its time, a precursor of the kind of megastructure and mega-environment (e.g., the Hancock Building in Chicago) that private industry would not begin building for decades. The best thing about the Pentagon in this respect is that it is horizontal, not vertical: city planners, corporate clients and architects still might ponder the usefulness of that lesson.

But not to praise the place overmuch on its 40th--just to give its architects and engineers their just due. The Pentagon today can hardly help but be a mind-numbing place. Fortunately it is rather easy on the eyes.