DEEP INSIDE THE PENTAGON, far behind those scenes occupied by the secretary of defense and his entourage of deputy and assistant secretaries, lie several separate, almost entirely unknown subcultures. In some of these specialized communities are the anonymous analysts who puzzle over their endless calculations; in others, the nuclear gurus who dream up doctrines and warfighting strategies; in still others, the professional briefers.

This last group is hardly the least important. Its inhabitants are commissioned to sell the wares of the powers that be. The dazzle or dreariness of their presentations can mean the difference between a weapon sold or rejected, a new idea advanced or squealched, a portrait of the Soviet threat embraced as truth or dismissed as flight of fantasy.

Standing out among these briefers, in a class all his own, is a soft-spoken, wiry, increasingly frail 54-year-old photo-intelligence analyst named John T. Hughes.

Few have heard of John Hughes, but in the coming months we're all likely to hear a great deal. For if a group of hawks on Capitol Hill get their way, Hughes will be unveiled as the secret superweapon that could produce political victory on the side of those trying to sell Ronald Reagan's and Caspar Weinberger's $274 billion defense budget to Congress and the American public. It may be an impossible mission, but if any one man can accomplish it, John Hughes many be the man.

Possibly without knowing it, you've heard of Hughes already. In October 1962, as the special assistant to the director of the Defense Intelligence Agency (DIA), Hughes examined a collection of overhead U-2 photographs and concluded that the Soviets were placing nuclear missiles in Cuba.

In February 1963, 31/2 months after the Cuban missile crisis was resolved, Hughes went on nationwide television for 90 minutes, at the request of then Defense Secretary Robert McNamara, and, with his pointer aimed at dozens of photos flashed before a large screen, demonstrated that the Russians had indeed removed those missiles from Cuba.

Nineteen years later, in March 1982, as deputy director of the DIA, a post he has held for the last 12 years, he appeared at a State Department press briefing, and -- at the request of then Secretary of State Alexander Haig, who was hellbent on proving Soviet and Cuban infiltration in El Salvador -- showed a set of overhead photographs revealing Soviet and Cuban influence in neighboring Nicaragua. As evidence, he pointed out a couple dozen (1950s-vintage) Soviet tanks, a few Soviet anti-aircraft guns and helicopters, "Soviet-style obstacle course," a "Soviet-style physical training areas" and new military garrisons "built along Cuban design."

But the product of John Hughes that the hawks on the Hill want to share with the rest of us is a briefing called "Soviet Military Trends and Capabilities." It's an updated version of a briefing that Hughes has been delivering around town, including to Congress' Armed Services Committees, every year for nearly a decade.

This briefing is classified at a level above Top Secret. It lasts three to four hours, with only a short break for breath. It consists of literally hundreds of overhead satellite and spyplane photographs of every military installation and weapon in the U.S.S.R., painting a frightening picture of Soviet military might, of a nation that appears to be brimming over with practically nothing but arms.

Several Republicans are urging President Reagan to declassify portions of the briefing and show it to the public. One look at those pictures, they say, and the resistance to the Reagan-Weinberger defense budget will crumble at once.

Says Sen. Ted Stevens (R-Alaska), chairman of the Appropriations Defense Subcommittee, "I don't think any normal person could listen to that briefing and not come out with his hair standing straight up on his head."

Practically everyone who has heard Hughes' delivery agrees that he is a brilliant briefer. "Air of confidence," "precision," "extremely skillful," "low-key," "straightforward," "the DIA's star salesman," "one of the best in the business. . . . a model for all briefers" are just some of the rave notices given by more than a dozen past and present Pentagon officials, intelligence officers and legislators of varying ideological stripes.

Hughes has been in the intelligence business for 30 years and has picked up five distinguished service awards along the way. His early background as a geographer -- he composed the large-scale maps of the world that the government freely distributed to universities after World War II -- prompted the Army to assign then-Lt. Hughes to its Pentagon photo-intelligence shop in 1953. And it was there that Hughes became, in his words, "hooked on photo-intelligence."

"When you look at that data," he explains, still with a gleam of wonder in his eyes, "you know that you're the first one looking at that image. When you're rolling that spool, you see something no one has ever seen before, and you know that you have to present that data very effectively because it could be critically important to the security of your country."

It's that "sense of discovery" -- and the knowledge that the virgin territory that he is uncovering is secret and important -- that has sustained his fascination over the years.

In 1957, he dropped his service rank and became a civilian analyst in Army Intelligence, then joined the interservice DIA when it was established in 1961. From the mid- 1950s on, the technology of the times and John Hughes' predilection for discovery meshed perfectly. The U-2 spyplane and then the Discovery satellite and its follow-on models were snapping thousands of photos; Hughes had plenty of spools to keep him engrossed.

And over the years, his briefings based on those spools have had a great, if largely untold, influence.

A former State Department official who still works in government recalls seeing the briefing in the mid-1970s, when it was commonly believed that as the age of detente bloomed and mellowed, the Soviets would cut back on their weapons programs. Each year the Pentagon would release statements claiming a continuing Soviet buildup, prompting nothing but cynical skepticism from State.

But then they saw the Hughes briefing. "It had a big effect on people then," the former official says, "in disabusing them of the idea that the Soviets are realigning their military plans to adjust the detente . . . The briefing was part of an intense program from within the Pentagon . . . to debunk detente -- and it was successful."

Robert Komer, Harold Brown's undersecretary of defense for policy in the Jimmy Carter years, saw the annual Hughes briefing with Brown and with several visiting NATO defense ministers. He recalls: "It was impressive to them. Yes, it was impressive to Harold Brown, it was impressive to me, because this was actual, hard, physical evidence." Whenever Brown or his assistants wanted an intellegence briefing, Komer remembers, "we would tend to ask for Hughes simply because he conducted himself more professionally than others."

The annual Hughes briefing, say those who have seen it, is very dramatic. He stands behind a podium, an assistant flashing one photograph after the next, as Hughes, pointer in hand, runs down all the clues that indicate a monumental buildup. His style is the drama of understatement -- straightforward, no obvious hyperbole, just the facts (some of the facts anyway), the overwhelming array, the panoramic display, the gruesome picture of a monstrous military powerhouse.

Says one DIA official, "It's the highly explicit character of photographs which gives his briefing the cutting edge. It simply leaps out at you... You look at it and say, 'Well, there it is, I can do everything but put my hands on it, so it's obviously true.' " But now that the shock that the Soviets are indeed building weapons has worn off, once the evidence is sifted from this new perspective and some questions are posed and some distinctions made, how does the substance of the Hughes briefing hold up? What does it suggest?

Many who have seen it note its impressiveness, but claim it doesn't really mean much. One quite hawkish former Armed Services Committee staffer who saw it several times says, "He shows you, for instance, an Oscar- class submarine, and it has X-number of cruise missiles in it, and he tells you how long it is and how wide it is, and the members of the committee say, 'Jesus Christ! Wow!' But so what? What does that mean?"

He continues: "If you ask him what it means, he doesn't answer. He deflects the question. He will categorically not do the interpretation . . . To me, it was a sideshow, it was theatrics."

William W. Kaufmann, a former special assistant to four secretaries of defense in every administration from Kennedy to Carter, says of the Hughes briefing, which he saw in the late-1970s, "It's like counting all the germs in the United States and saying we're all about to be dead."

In short, in the lingo of intelligence analysis, it is not a "net assessment." It doesn't compare the United States versus the Soviet Union, or NATO versus the Warsaw Pact; it just looks at everything the Soviets are doing, in the absence of context. Several analysts observe that a similar photographic array of everything the U.S. military is doing would, in the words of one, "look pretty damn scary, too."

Even the pictures of all those weapons sometimes carry misleading impressions. "One might say," says one former high-ranking DIA officer, "that the facts which John has selected are the facts which are alarming."

Richard Stubbing, former deputy chief of the Office of Management and Budget's national security division, remembers that two years ago, the Hughes briefing "said that the (new Soviet) T-80 (tank) was going to be the sine qua non of tanks." Since then photos of the T-80 have appeared, and "we now know that the tank isn't much different from the T-72, which is not the world's greatest."

Similarly, the Soviet's new Ivan Rogov amphibious boat was played up in Hughes' briefing for a few years as a sign of the Soviets' growing ability to "project power" and intervene in the Third World. "But the thing is only half the size of (U.S. amphibious boats), and they only have one of them," says Stubbing. "What in the world will the Ivan Rogov do if it . . . runs into any kind of opposition? The entire Soviet surface Navy, after the first hour of combat, is a joke."

Other analysts note that Hughes makes much of the thousands of surface-to-air missiles (SAMs) all over the U.S.S.R. Yet, says one former high-ranking CIA officer, "He'd point out lots of SAMs, but whether they were highly susceptible to jamming or spoofing tended not to be part of the presentation."

Then there is a still more fundamental question, posed by a onetime boss of Hughes', Lt. Gen. Samuel Wilson, former DIA director and, before that, the U.S. military attache in Moscow.

"I am concerned," says Wilson, "that many of the people who listen to the 'hard facts' and survey the arithmetic don't get far enough into" analyzing "the sources of Soviet conduct." Wilson believes that much of the U.S.S.R. military buildup is a product of the Soviets' preoccupation with China and West Germany, their memory of having invaded three times this century -- especially their "nightmarish" obsession with their losses during World War II, which Wilson has described as "a wide vivisectional scar from . . . chin to . . . crotch" which each Soviet citizen scratches every morning "until it hurts," and with which he goes off to work in pain every day.

"Let's say you know a fellow who had an unhappy childhood and . . . has been deeply scarred by his past," Wilson says. "He is gripped with paranoia, he's a gun nut, his house is full of the And you are trying to . . . live peacefully with him. If you look only at his guns . . . and figure anytime you go around him you, too, had better be armed to the teeth, lest he gets a drop on you, then I don't think you're addressing the situation fully; you may even be complicating it."

This is not to say that the fellow isn't dangerous, says Wilson, "but -- to stretch the metaphor back to the Soviets -- he's so xenophobic and so suspicious that anything you say or do is likely to be misinterpreted and cause him to get even possibly further trigger- happy . . . Simply counting the guns of a fellow who is paranoid and living in a house full of guns is not the whole story."

In short, the Hughes briefing -- and that style of analysis generally -- says nothing about the meaning of the Soviet buildup or about what the United States should do in the face ot it. Nor, for that matter, does Hughes claim to be exploring this dimension. He claims only to be showing a lot of photographs. He doesn't even explicitly state any conclusion, but, in the words of one official who has seen the briefing several times, "leaves it to the imagination of the listener."

The conclusion that one is supposed to walk away with, however, is not too difficult to miss. Those on Capitol Hill who are trying to use the briefing as a political tool in the selling of the Reagan-Weinberger defense budget are interpreting it as many in the DIA -- which commonly supports the various needs of the secretary of defense -- obviously have meant it to be interpreted.

Says former DIA Director Gen. Wilson: "If the Hughes briefing were to serve as a magic wand that simply causes everybody to say, 'This is horrible, we've got to approve whatever (the administration) wants,' and left it at that, I would regard it as grossly incomplete."

John Hughes is clearly a master at his trade; he's an artist, a pioneer. But the senators and congressmen who want him unleashed on the American public are playing political games. What Hughes has to say hardly speaks at all to the questions of how much we should spend on defense or of what kinds of weapons we should buy.