For weeks little doubt has been expressed by Defense Secretary Casper W. Weinberger or other top administration officials about the damage that Marine guards caused at the U.S. Embassy in Moscow.

The United States has suffered "a very great loss," Weinberger said in a March 28 interview on Cable News Network, the day two guards were accused of allowing Soviet intelligence agents into the building. On April 16, he attacked the Soviets for running "a massive espionage campaign" that extended into "some of the most sensitive rooms in our embassy in Moscow."

Last Friday, after the Marine Corps dropped all charges against the second embassy guard implicated in the case, it was clear that the government would have difficulty proving the case that Weinberger and other administration officials were making a few weeks earlier. Cpl. Arnold Bracy, the accused Marine, and his lawyers said it simply: The alleged embassy entries did not occur.

Defense Department spokesmen said yesterday that Weinberger stood behind his statement and that the Soviet incursions were "a matter of record." His rhetoric was not overstated, a spokesman said.

"Anytime he discussed it, he was dealing with potential based on the information he had," Robert B. Sims, Weinberger's chief spokesman, said in an interview. "He hasn't changed what he said.

"We're not sure we fully know what the damage is or if we ever will," Sims added. "All we can talk about is potential seriousness based on what the investigation showed."

What may have happened, said defense lawyer Lt. Col. Michael L. Powell, was a "fantasy" created by members of the Naval Investigative Service (NIS), the largely civilian police force within the Navy Department charged with investigating crimes by members of the Navy and Marine Corps.

And what created the James Bond-like fantasy? "When you see a seven-page confession, you tend to believe it," said Powell, head of a team that represented Bracy.

It was only when the defense lawyers entered the case that someone began to complain that Bracy had consistently recanted the statement and that it contained none of the initialed annotations or corrections it was supposed to have.

Lawyers for Sgt. Clayton J. Lonetree, the first Marine charged, had made similar claims since Lonetree's arrest. But because Lonetree remains jailed on espionage charges based on his own statement, his credibility may have seemed in question.

Bracy, 21, was cleared of all charges. Yesterday, on ABC-TV's "Good Morning America," he repeated his contention that NIS agents tricked him during interrogation.

Moreover, as Bracy's lawyers have pointed out, he is not alone in making such statements. Not only has Lonetree made such allegations, but at least three other former Moscow embassy guards who testified at Bracy's pretrial hearings said that NIS agents attempted to threaten them and get them to sign false statements.

As in Bracy's case, many were taken off-base to a nearby motel where they were questioned for hours, often while hooked up to polygraph machines, and were told that a statement was the only way out. It was part of a worldwide NIS investigation that involved more than 100 agents and was code named Cabin Boy.

The problem with Cabin Boy was that it was aimed at quickly assessing the intelligence damage in Moscow, not producing evidence that would stand up in court, according to a Justice Department source. As a result, Justice, usually the key player in espionage cases, wants to play only an advisory role in the inquiry.

That leaves two pending criminal cases against the Marine guards, the remaining espionage charges against Lonetree and charges of fraternization with Soviet women involving Staff Sgt. Robert S. Stufflebeam, 24, who was questioned by the same NIS agents who interviewed Bracy. Stufflebeam was notified yesterday that he will face a general court-martial on the charges, his family said.

"I don't like to say, 'I told you so,' but I did," said Rep. Jim Bates (D-Calif.), who for years has argued that the Naval Investigative Service is inept. "It continues to fumble and bungle the major issues given to it," he said.