On Friday, Aug. 17, a U.S. spy satellite in orbit over the Caribbean trained its high-powered lenses on a tract of rugged country near the southern coast of Cuba. The pictures transmitted electronically to the ground were examined by photo interpreters in offices scattered around Washington. They revealed the tanks, artillery, trucks and tents of a military unit on field maneuvers.

The photographs were of grave significance, for a reason known only to a handful of U.S. intelligence officials: A few days before, they had been tipped off that a Soviet combat unit stationed near Havana planned maneuvers across the island at the time and place where the satellite cameras trained their lenses for high-resolution zoom shots.

On Aug. 20, another satellite mission over Cuba found the maneuver area deserted and the heavy artillery equipment parked once more in two inconspicuous areas a few miles west of Havana that are the suspected base camps of a Soviet brigade.

The pictures of the Russian guns of August, together with confirming data that still are secret, ended an internecine argument of long standing among U.S. intelligence agencies and officials. Most of the skeptics and the doubters now agree that a Soviet combat force of several thousand men has been stationed in Cuba for many months -- perhaps for many years.

This unavoidable conclusion has touched off a new Soviet-American confrontation, endangered the embattled strategic arms limitation treaty between the superpowers, and has posed a new challenge to the sagging political fortunes of President Carter.

Last Friday afternoon, three weeks after satellite photographs ended an argument and began a new crisis, a somber Carter appealed to the nation from the White House for "calm and a sense of proportion" in equal measure with "firmness and strength."

In the public metaphor of high officialdom, the problem of finding a unit of 2,000 to 3,000 Russian soldiers on a Caribbean island of 10 million persons and 190,000 Soviet-equipped Cuban troops was a "jigsaw puzzle" of excruciating difficulty. While there is no doubt that the challenge was formidable, it is also true that only a few people and a tiny fraction of American intelligence resources were devoted, until recently, to fitting together this unexpected and unwelcome picture.

The origins of the Soviet effort are obscure, but top officials of several U.S. intelligence agencies suspect that the starting point was the Russian buildup of 1962 -- 17 years ago -- when Moscow put offensive missiles, bombers and about 20,000 first-line troops in Cuba.

The resolution of that missile crisis, perhaps the most dangerous superpower confrontation of the nuclear age, required the removal from Cuba of the Soviet offensive weapons and of all Soviet forces associated with the missiles and bombers. According to those who have studied the diplomatic exchanges and understandings -- some of which have never been made public in full detail -- there was no agreement covering Soviet ground forces in general.

Nor was there much attention to the subject then or in most of the years since. An official who recently reviewed the record of highly confidential U.S. deliberations and action in the missile crisis, a stack of documents several inches thick, could find only 1 1/2 pages which made reference to Soviet ground troops. A top Central Intelligence Agency official said last week, "Soviet ground forces in Cuba have not been a priority item . . . they weren't considered a threat to the United States."

It was well known in Washington and no secret in Havana that hundreds of Soviet military advisers -- 1,500 by one estimate -- were left behind in 1962. Beginning in the early 1970s, there were also well-documented reports that some of these troops were on hand to guard and operate a large and highly sophisticated Soviet electronic eavesdropping station established on the Caribbean island.

On the basis of retrospective hints, high officials now believe it is plausible and possible that a Soviet ground combat unit has remained in Cuba, under the nose of the United States, since the buildup and the withdrawal of 1962. The evidence is slender and inconclusive, however.

Beginning at least a decade ago, U.S. intelligence received periodic and fragmentary reports of Soviet ground force units of a few thousand men in Cuba. These reports were not taken at face value and raised no alarm signals at the top of the government. CIA officials said it is doubtful, in fact, that they ever got to the top.

One reason was that in the late '60s and early '70s, the intelligence community (like the rest of the government) was obsessed with Vietnam. Intelligence "assets," both human and technological, were directed at that part of the world; there was little left over for intelligence operations aimed at Cuba.

By the mid-'70s the Vietnam adventure had ended but there was still interest in Cuba. The National Security Agency picked up references to a Soviet "brigada" in Cuba in 1976. But nothing was done about it; the information, in effect, was ignored.

Several explanations are now being offered.

First, the analysts didn't know what to make of references to a brigade. It is an aberrational form of military unit in the Soviet army. Most Soviet ground forces are organized into regiments and divisions. Only four "brigades" were known to exist in the entire 1,800,000-man army -- a ceremonial unit in East Berlin, a unit in East Germany, and two units in Mongolia, whose functions are still unknown.

So the intercepted "brigada" chatter out of Cuba set off no alarm in the intelligence community in 1976. Analysts assumed that somebody was mistaken or confused.

Second, the Soviets went to unusual lengths to conceal the presence of their ground unit among the Cubans. The Russian colonel in charge and his men, who are believed to serve tours of two to three years in Cuba, were never mentioned in public by either Moscow or Havana.

The brigade was split between two separate locations resembling Cuban camps a few kilometers from one another near Los Palacios, 60 miles west of Havana, rather than camped together in recognizable Soviet style. The unit maintained a high degree of radio silence and only rarely conducted maneuvers, according to American officials.

Third, there was very little U.S. interest in the subject. Without indications of "sufficient weight to warrent a presumption" of a Soviet combat unit in Cuba, "we weren't looking for it," according to presidential national security affairs adviser Zbigniew Brzezinski. Until this summer, the National Security Agency, which is a very large organization, had only one analyst assigned fulltime to material from Cuba.

The first break in the process of discovery came in early 1978, when "a happy accident" brought the U.S. intelligence within a few days two specific pieces of information about a Soviet brigade in Cuba. An intensified study was ordered. It produced photographs of modern Soviet military equipment deployed in camps near Los Palacios and photographs of a Soviet training mission at a Cuban gunnery range in the western part of the island.

From this evidence, officials at the Defense Intelligence Agency and the Central Intelligence Agency drew the wrong conclusion. They ruled that the military equipment was assigned to Cuban, instead of Soviet, forces and that the "brigade" bivouac areas were Cuban camps. Some lower-level U.S. intelligence officials strongly disagreed with that assessment.

Late in 1978, U.S. concern over the arrival in Cuba of modern Mig23 combat aircraft prompted the first U.S. spy plane flights over the island since Carter called them off in 1977 as a gesture of goodwill to Havana. The Mig23 incident heightened U.S. interest and surveillance, but the overflights were not continued on a regular basis.

In March this year, a White House memo signed by Brzezinski ordered CIA Director Stansfield Turner to assess the size, location, capabilities and purposes of Soviet ground forces in Cuba. One of the practical results was to send NSA'S lone Cuban analyst back through the agency's voluminous computerized files for bits of pertiment information. After a second White House memo a month later, other intelligence organizations joined the search.

By mid-June the NSA analyst completed a study which, in retrospect, was a landmark in the search for the Russian brigade. For the first time an accumulation of evidence argued convincingly that, at a minimum, a Soviet brigade headquarters had been established in Cuba.

The study set off a fierce dispute within the intelligence field, in part because of its implications for U.S. policy. NSA and Army intelligence argued that a combination of photography, signal intelligence and a rare bit of human intelligence pointed unmistakeably to the presence of a clandestine Soviet brigade. According to informed sources, CIA, DIA, State Department, Air Force and Navy intelligence chiefs disagreed.

The basic information was available to all the agencies, and thus the issue was one of interpretation and evaluation. With Carter signing the long-awaited SALT II treaty with Soviet President Leonid Brezhnev in Vienna in mid-June and the administration preparing for a battle royal over Senate ratification, the political implications of belatedly discovering Soviet combat troops in Cuba were grave. According to a senior intelligence officer, his superiors said repeatedly, "We've got to save SALT, whatever you do keep that in mind."

An early July review of the intelligence did nothing to resolve the deadlock. NSA and the Army were even firmer in their insistence that there were strong and precise indications of a Soviet combat force. Other agencies were unmoved. The compromise result, engineered by CIA'S Turner, was a mid-July agreement that a Soviet force was present as a separate unit, not part of an advisory group. But there was no agreement on the size, organization or mission of the Soviet force.

During the July deliberations the Army argued that the official report should take note of the purposes of the Soviet unit, including the possibility that its mission is to guard existing or potential nuclear weapons. According to an official present at the coordinating meeting, Turner telephoned a high Army officer to argue against any such statement, even as a dissenting view.

"We heard only one end of the conversation, but that consisted of firm statements that Army was being unreasonable and that it should fall off . . . He [Turner] in effect ordered them to cave in" and the Army did so, the participant reported. A CIA spokesman, asked about the incident, said Turner had intervened to keep "gratuitous speculation" out of the coordinated intelligence report.

One result of the mid-July coordinated report was a memo from Carter to Turner directing stepped-up intelligence surveillance to determine the nature and purpose of the Soviet ground unit, if one in fact existed and authorizing a diversion of resources from other areas of the world if necessary. A very heavy effort involving satellite photography and other highly sophisticated technology was mounted. The same concentration of effort, if carried out on a worldwide basis for a year, according to an informed official, would cost about $100 billion, nearly as much as the entire Department of Defense budget.

Another result of the intelligence controversy and compromises of July was a series of leaks to members of Congress and news organizations. On July 11, Sen. Richard Stone (D-Fla.) questioned the Joint Chiefs of Staff about Soviet forces in Cuba, and on July 15 he began a series of public charges about Soviet military activities there. On July 20, ABC News reported that Soviet combat forces were in Cuba. These reports attracted little public attention, and were denied in essence by official spokesmen.

Secretary of Defense Harold Brown, appearing before the Senate Foreign Relations Committee on July 17, reported that there was no evidence of a "substantial increase" in the size of the Soviet military presence in Cuba over the past several years. He added that apart from the Soviet military advisory group, "our intelligence does not warrant the conclusion that there are any other significant Soviet military forces in Cuba." The same language was used by Secretary of State Cyrus R. Vance on July 27 in replying for the administration to a letter from Stone.

These cautiously hedged statements did not define such terms as "significant" nor did they reveal that a crash effort had been mounted at presidential directive because of strong indications of a Soviet brigade in Cuba. Stone called the Vance letter "a whitewash." Another official said the Vance-Brown statements contained part truths which are commonplace in public statements on controversial intelligence studies.

In the early part of August, the intelligence drive paid off with a report that the Soviet brigade planned maneuvers across the island near the middle of the month. Also in early August, perhaps in response to such findings, Carter directed, through Brzezinski and Turner, that intelligence on Soviet forces in Cuba be stepped up to "highest priority."

It was this effort that paid off on Aug. 17, in a fraction of a second and the snap of a shutter high above the Cuban countryside.