At three in the morning scores of officers are awake at the Pentagon. They sip coffee, exchange teletype messages and hope, with the grudging perversity of boredom, that something will go wrong.

Tonight, in the Air Force security center, six security officers, as usual, are wearing revolvers and sitting around a huge panel of red and green lights. The panel shows the status of each Air Force office door in the Pentagon. A red light means a door is open, a green that it's closed. At 3 a.m. only four lights are red on the board and each of these offices has been cleared in advance. Almost no one works late at the Pentagon; even the escalators are turned off at 5:30 p.m. every day.

Suddenly, the board flashes an unexpected red light.

"Where's that?" asks the sergeant in charge.

"Three. Easy 243."

Two large soldiers grab walkie-talkies on their way out of the room.

The sergeant looks up the numbers in a book and slips pieces of pink paper into a time-clock to record the incident for any future investigation. After about thirty seconds the sergeant speaks again. "What's that tied to?" he asks one of the men.

"That's a vault," comes the reply.

"Check the records," says the sergeant.

More papers are shuffled. His next in command flicks a switch to see if the red light will go off. No, it stays on.

Room Easy-243 is located three floors up on the far side of the Pentagon and the two men with the walkie-talkies have taken orange electric carts, equipped with bicycle bells, to get there. Now their voices are coming over the radio, almost incomprehensible: "We're in the area."

A minute goes by and a static-filled voice says, "No cause for the alarm can be determined." The sergeant asks them if they've reset the alarm.

"Affirmative," says the voice. The green light comes back on the board. The incident is over.

When the two men come back one of them mutters "Boy, that f - cart was slow as s -." It turns out the two men, revolvers at the ready, couldn't have gotten in the office to see if anyone, indeed, had broken in and closed the door behind them, because the office had a combination lock and their security detail had only been given keys. Someone asks one of the two men if it bothered him that he couldn't investigate the office. He shurgs and says, "You know what's nice about the night shift here is that stagger the hours so when we get off we miss rush hour."

Elsewhere in the Pentagon, more exciting security is going on all night long.

In the National Military Command Center about sixty men, including a representative from the CIA, baby-sit the hot line to Moscow. (Most people believe, incorrectly, that the hot line terminal is in the White House.)

Their open lines guarantee instant communication with American armed forces throughout the world, including SAC bombers and nuclear submarines. Duty officers even monitor the overnight locations of the Joint Chiefs of Staff - just in case.

The mood here, a spokesman says, ranges from "boredom to stark terror." The terror comes from the real possibility of having to make life-or-death military decisions, which could escalate into war. There is no margin for error.

"If there's a crisis, we do certain things and quickly forget whether it's even night or day," says one veteran of some twenty military emergencies.

Somewhere in the bowels of the building each branch of the military also maintains a twenty-four-hour "operations and readiness" center.

The Air Force center is typical. It is a Dr. Strangelove world inside a long oval room, with a row of clocks showing the time in major world capitals, and above everything else a line of elevated command areas separated from the room by sloping glass windows.

Here about thirty men monitor every Air Force base in the world, and have at least two direct lines to the White House. But even the Pentagon needs a back-up system, and a parallel Air Force crew monitors the Pentagon room from a similar facility at Ford Ritchie, Maryland.

"The Pentagon is not a 'hardened' site," explains a major."If we get nuked, we become part of the fallout. So if we just disappear in a flash, the Fort Ritchie people will know what we were working on and can continue."

This may be the only place in Washington at 3 a.m. where "nucked" is used in routine conversations.