The recruitment of a Marine guard at the U.S. Embassy in Moscow was a major goal of Soviet KGB secret police who considered the guards "the line {they} could never breach," a witness in the spy trial of Sgt. Clayton J. Lonetree yesterday quoted a KGB defector as saying.

The trial also delved into Lonetree's high school days, with one of his former teachers showing the court a notebook of Lonetree's bearing a large swastika, the Nazi symbol, and the words, "Hitler lives."

Author John D. Barron, introduced as an expert on the KGB, was questioned by government prosecutors on a hypothetical recruitment and entrapment case matching Lonetree's, a former Marine guard at the Moscow embassy. Lonetree, 25, is being court-martialed at the Quantico Marine Base on 13 counts of espionage and related charges and faces a maximum life sentence if convicted.

According to Barron, who has written three nonfiction books on the KGB, the methods used and risks taken by the KGB in the hypothetical case would indicate that the KGB considered this recruitment a "principal success story."

Barron said he was once told by a defector, KGB Maj. Yuri Nosenko, that the KGB had never been able to infiltrate the Moscow embassy because the "Marines were the line we could never breach." The events leading up to Lonetree's arrest on suspicion of espionage last December were a textbook example of the sexual entrapment, recruitment and running of an American citizen by Soviet agents, Barron testified.

While stationed in Moscow in 1984-86, Lonetree had an affair with a Soviet woman, Violetta Sanni, whom he called "my darling" in letters produced as evidence.

As Barron told of KGB use of sexual entrapment, Lonetree was seen to wipe his eyes. Defense counsel William M. Kunstler said later that Lonetree had wept and whispered, "I thought she loved me."

Sanni introduced Lonetree to Alexei (Uncle Sasha) Yefimov, a KGB agent, the trial has been told. Sasha and Lonetree met in Moscow and during Lonetree's later duty in Vienna, which Barron called a major center of KGB activity.

Lonetree reportedly had several meetings with a high-ranking KGB officer in Vienna, receiving more than $3,200 for sensitive information about the embassy.

"The willingness of the KGB to continue payments and to involve a senior Soviet officer {indicates} the importance the recruit had for them," Barron said.

"The factors demonstrate the KGB perceived the operation to have . . . establish{ed} a clandestine relationship with a willing agent who had proven amenable to the dictates of the KGB, {and} who had provided information."

Lonetree's defense has stressed what it has characterized as the poor quality of information he passed to the KGB. Barron said the KGB draws recruits into a clandestine liaison by encouraging them to pass information when both parties know this to be illegal.

He added that the Soviets do not pay for worthless information. Reacting to defense attempts to portray Lonetree as naive, he said the Marine could have been naive or stupid or "willingly . . . a traitor to his country."

He said of the alleged "loneliness" of a young Marine in Moscow: "When you put on the uniform you are expected to give up your life for your country, so I don't think it's unreasonable to expect him to go without affection."

The trial was closed for about two hours' testimony from a secret witness, identified in court only as John Doe. Kunstler said the defense was not allowed to cross-examine this man, the bulk of whose testimony was corroboration of Lonetree's incriminating statements, which have been admitted as evidence.

June Dahl, who taught Lonetree high school history for three months in 1977, testified that a class notebook he used as a 16-year-old student at Johnson High in St. Paul, Minn., was "embellished" with swastikas, anti-Semitic statements and "what I call crude poetry." Scrawled on the book's yellow cover were the words, "Holocaust all a lie."

Under cross-examination, she said the book also contained the hammer-and-sickle symbol of the Soviet Union, "a discussion of apartheid and how awful it is," and a list of members of the Socialist Workers Party of the United States.

Lonetree was a "very quiet individual," Dahl said. He "didn't talk to the other students and they didn't talk to him."