By the end of last week, two things were clear.

First, despite the publicity, despite the handwriting in the Senate and the president's concern over this "very serious matter," no one in the military or intelligence communities regarded the Soviet troops in Cuba as a military threat to the United States. The mysterious "brigade" is an insignificant military force, no larger in fact than the forces maintained by the United States inside Cuba -- at the Guantanamo Bay Navy Base.

Second, regardless of its military insignificance, the "brigade" was a political issue ripe for exploitation by opponents of the SALT II pact and by liberal senators, such as Foreign Relations Committee Chairman Frank Church (D-Idaho), who are running for reelection in conservative constituencies.

Less clear was the lesson for the U.S. intelligence community. Did its failure to turn up the brigade sooner demonstrate serious weaknesses in its capabilities? There was much disagreement among the analysts themselves on that point.

It was also not clear how Carter could end the affair. The Russians could easily resolve his political problem by withdrawing the brigade. But that may not be their response.

A few hours before Carter addressed the issue on Friday, two Soviet government analysts, on a visit from Moscow, were having breakfast in Washington with a group of reporters and editors.

This issue of the brigade, one of them said, was a "trifling" matter -- purely a "domestic political issue for the Americans."

Why make such a big fuss about Russians 90 miles off American shores, asked the other, when there are American troops in Norway which directly abuts the Soviet Union? He could have underscored the point, but didn't, that the Americans also have 4,700 troops on Russia's southern flank in Turkey and 2,200 in Cuba itself at Guantanamo.

He said the Soviet military in Cuba was there to train Castro's troops in how to use new technology equipment and traced its origins back to the early 1960s.

The current uproar, he said was a provocation and purely a "domestic political issue" for the United States. He suggested it had been raised for several possible reasons. The U.S. government wanted to extract new concessions from the Soviet Union as the price of SALT II; or SALT II opponents had forced the issue out as a means of delaying Senate consideration. He also proposed that the U.S. intelligence community had come up with old information to embarrass Cuban President Fidel Castro on the eve of the nonaligned nations summit in Havana.

In any of those instances, the Soviet visitor insisted, it would be difficult to see the removal of Russian troops that he insisted had been in Cuba for years without any U.S. concern.

If SALT II were endangered by the troop presence in Cuba, he was -- unhappily -- prepared to see the treaty delayed.He added that he believed had the Cuban issue not developed, something else would have emerged to sidetrack the treaty.

Whether this will turn out to be the official position of the Soviet government after its representatives discuss the troop issue at the State Department this week remains to be seen.

There are some U.S. officials who fear the anger about the Soviet brigade will spill over to SALT II itself, imperiling its chances of approval.

Reelection realities have already heated up the issue, with Sen. Church, the target of a conservative "Anybody But Church" political action committee in Idaho, suddenly putting some distance between himself and SALT II by stating there is "no likelihood whatever that the Senate would ratify the SALT treaty as long as Russian combat troops remain" in Cuba.

To save SALT II, President Carter -- who fervently wants the treaty to be ratified -- must find a way to persuade the Soviets that their troops in Cuba must be declawed somehow, or removed, and at the same time convince wavering senators that the same troops are not enough of a military threat to justify voting against the treaty. This is why Carter and his allies are trying to dig a firebreak between SALT II and the troops, as the first of many strenuous tasks forced upon them by the Soviet military presence in Cuba and the failure to detect it sooner.

"We are not dealing with a strategic crisis nor are we dealing with a direct and overt military threat directed at the United States," said national security affairs adviser Zbigniew Brzezinski in digging that firebreak before a group of out-of-town editors invited to the White House Friday morning.

"The issue is of a different nature," he said. "It involves the stationing of Soviet combat forces which at the same time is pursuing an internationally active revolutionary role. It is more, therefore, of a political problem."

Perhaps closer to the current situation was the flurry in 1970 over the prospect that a Soviet Navy shore facility was being built on an island in Cuba's Cienfuegos Bay. The construction of a soccer field was the hint at what was afoot and the Nixon administration protested to Soviet Ambassador Anatoliy Dobrynin.

According to the account published by former president Richard M. Nixon in his autobiography, "RN," Dobrynin was told the U.S. government had told the press "we did not yet know whether there actually was a submarine base in Cuba . . . in order to give the Soviets an opportunity to withdraw without a public confrontation."

As Nixon relates it, two weeks later Dobrynin returned with a note saying the Soviets were doing nothing that violated understandings made at the time of the 1962 Cuban missile crisis and "after some face-saving delays, the Soviets abandoned Cienfuegos."

Late last year, when there were allegations raised that the Soviets had supplied nuclear-capable Mig23 aircraft to the Cuban air force, the Carter administration raised the question with the Soviets. The response, in a note, was that the planes were not equipped to carry nuclear bombs. The aircraft remained in Cuba and the controversy subsided.

More recently, there were reports that a Soviet naval fleet was heading toward Cuba with submarines capable of carrying nuclear missiles. In this case the fleet avoided landing at Cuba.

As Brzezinski told the reporters Friday, however, the current flareup is over "more ... of a political problem" than a military one. Thus the solution more likely will be a political rather than a military one.

The Cuban issue raises some tough problems for the Soviet leadership if they are looking to make a concession. Unlike 1962, when they bowed to U.S. pressure and removed medium-range missiles, heavy bombers and their 20,000-man division, Soviet strategic power today is considered the equal to that of the American force. Thus the Russians stand to lose some face if they appear to give in.

That perception will be even greater because unlike the Cienfuegos and Mig23 incidents -- which could have involved violations of the 1962 understandings -- there is nothing between the U.S. and Soviet governments that would bar the stationing of combat troops in Cuba.

Finally, the Soviets must consider the reaction from Cuban President Castro to any move they make.

When the Soviets in 1962 decided to remove their missile forces, Castro reportedly "was furious," according to a recently published account of the crisis by David Detzer. Detzer goes on to recount that Castro "refused initially to give up the I128s (bombers) which the Russians had turned over to him."

Detzer wrote that only continued threats of an American invasion convinced Castro he had to return the planes to the Soviet Union.

The Soviets have already toyed with one possible area for trading in the current situation. In a Moscow broadcast Friday beamed in English to the United States, it was noted that the American Navy base at Guantanamo Bay in Cuba, with its 2,200 men, is "a dagger pointed at the heart of the young republic."

Military leaders from the Joint Chiefs of Staff down agree that the Soviet troops pose no direct threat to the United States or its Caribbean allies, partly because the Soviets have not deployed the ships and planes to Cuba for taking their troops anywhere to create mischief.

Known hardliners, including those no longer inhibited by active duty status, are agreeing with Brzezinski's assessment that the Soviet troops present no "direct and overt military threat."

Retired Army Lt. Gen. Daniel O. Graham, a frequent spokesman for the hardline American Security Council, said that the direct military threat from the Soviet brigade would be the least of his worries if he were still advising the joint chiefs as director of the Pentagon's Defense Intelligence Agency, a post he held from 1974 to 1976.

"Of course they're not going to invade Miami," Graham scoffed. "What I would tell the chiefs, if I were still there, is that the" 3,000 troops "are a strong indication that the Soviets have at least contingency plans to put nuclear weapons in Cuba" because they traditionally use such outfits for that purpose.

Another retired intelligence leader, who declined to be quoted by name, said "damned if I can see that it makes any military difference. An organized formation of 3,000 men poses no possible menace to the U.S. proper. This is a nonevent" militarily; "it's Republicans vs. Democrats."

While such assurances undercut any "Russians are coming" scare talk that may come out of this latest U.S.-Soviet confrontation over the military use of Cuba, they do not answer the tougher question of how the U.S. intelligence agencies failed to detect the Russian combat brigade for so long.

Once more American intelligence was found wanting at the very moment the president was trying to persuade Congress and the country that the Soviets would be caught if they cheated on SALT II'S limits on nuclear weapons.

One intelligence agency's warning against sending the USS Pueblo off North Korea to eavesdrop got mislaid in the Pentagon shortly before the ship was captured in 1968. The congressional hearings on the Israeli attack on the USS Liberty spy ship in 1967 revealed that a lot of information went to the wrong place and much of it was never read.

Pentagon civilian leaders complained the intelligence agencies failed to warn them that the Yom Kippur War of 1973 was coming. The Central Intelligence Agency did not foresee the deposing of the shah of Iran in time to do much about it.

And now, after first denying Sen. Richard Stone's assertions that there were Soviet combat troops in Cuba, the Carter administration has admitted he was right after all; that U.S. intelligence had finally found combat troops that may have been in Cuba since 1962.

Some senators were quick to assert that if U.S. intelligence agencies could not find Russian troops off the American coast, they certainly could not find strategic missles hidden in Russia. "If they can't find the nose of Russian troops, how can they count the noses of Russian missiles?" asked Sen. Henry M. Jackson (D-Wash.), a leading critic of SALT II.

Secretary of State Cyrus R. Vance, Sen. Alan Cranston (D-Calif.) and intelligence officials themselves insist that is a vast difference between counting noses and counting giant missiles, and that the Cuban experience has no relevance to the SALT verification issue.

Nevertheless, inside the intelligence agencies, there is bitterness and dismay over the latest humiliation.

Several analysts at the working level said that even when they sent evidence about the Soviet combat force it was either ignored, suppressed or misrepresented by Carter administration officials. The Defense Intelligence Agency, they said, refused to accept the evidence until the very end of the internal debate. One analyst claimed "Vance was lying and we knew he was lying" when he wrote Stone that there was only a Soviet command structure for a brigade that had not been fleshed out with troops.

Other intelligence officers asserted that their superiors at the top are asking them to do more with less, virtually guaranteeing gaps, especially in low priority areas like Cuba. Partly through imposed reforms and partly through lack of money and people, officers said, intelligence from places like Cuba is gathered almost exclusively by mechanical means -- primarily satellite photography and electronic ferreting by ships and planes flying around, but not over, Cuba.

"There's just no substitute for having a guy who can walk along a fence and tell you whether the troops inside the wire were speaking Spanish or Russian," said one U.S. official. He said this lack of "human intelligence" handicapped the effort to find out what was going on in Cuba.

Also, to the dismay of some professionals within the intelligence community, CIA was so shaken by the disclosures of its "excesses," such as plot to kill Castro, that it made no systematic attempt to interview Cuban refugees about Soviet activities on the island.

Given those difficulties, some administration leaders reject the charge that taking so long to find the Soviet brigade in Cuba represented an "intelligence gap." On the contrary, these officials contend, finding troops that the Soviets went to such lengths to hide really is an intelligence coup for the United States.

Beyond the argument over "intelligence gap" and its impact on SALT II, many present and former intelligence executives fear that the way the troop evidence was finally disclosed leaves little room for an acceptable U.S.-Soviet compromise.

"We haven't left them much of an exit," was the way retired Lt. Gen. Samuel V. Wilson, former head of the Defense Intelligence Agency and a longtime student of the Soviet military, described the Cuban situation as he sees it today, from outside government.

Inside government there is support for Wilson's concerns that the public nature of the dispute and the heat attached to it made more difficult the chances for the Soviets to agree to an accommodation.

At the White House State and Defense departments, officials are tight-lipped about what they hope to accomplish.

"We're not going to talk about what we want," one said, "because if conditions become public they almost automatically become unacceptable."