John Barron, once a Navy intelligence officer and author of three books on the KGB, testified in Quantico, Va., in the espionage court-martial of Marine Sgt. Clayton J. Lonetree, that the Soviet spy agency believed Marine guards at the U.S. Embassy in Moscow were "the last line" of defense in protecting U.S. secrets. Lonetree wept for the first time during the five-week court-martial when Barron detailed the KGB's extensive use of sex as an espionage tool. Lonetree is charged with passing national security secrets to the Soviets while a guard at the U.S. embassies in Moscow and Vienna. He was introduced to the KGB after falling in love with a Soviet woman.