WITH THE sentencing of Sgt. Clayton Lonetree to 30 years' imprisonment, a major phase of the Moscow Marine scandal has come to an end. From the beginning, Sgt. Lonetree was the key figure. His guilt has now been established. His offenses are serious. A member of an elite unit, the embassy guard was the first Marine in the 212-year history of the corps to be tried for espionage. Now a military jury has found him guilty of acts he first confessed last December: he had an affair with a Soviet agent and under her influence gave to the KGB photographs of U.S. intelligence agents, embassy floor plans and more than 100 classified documents. In exchange, he received money from the Soviets.

Disgraceful as this case is, six months ago it appeared to be much worse. Four Marine guards had been implicated, the entire Moscow guard contingent of 28 young men had been returned to Washington, and charges were being made that the physical security of the embassy itself had been breached. But this was not proved. No evidence was produced that substantiated the tale of guards letting KGB agents into the building at night, guiding them to offices and code rooms and standing by while bugs were placed and secret material compromised. Charges against one Marine, Cpl. Arnold Bracy, have been dropped, and those against two others, Sgts. Robert Stufflebeam and Kenneth Kelliher, relate to contacts with Soviet women but not to espionage.

Questions remain concerning the investigation, which was conducted by a civilian arm of the Navy. Under what circumstances, for example, did both Sgt. Lonetree and Cpl. Bracy confess to letting the Soviets into the embassy? Did justifiable concern about embassy security in Moscow -- compounded by revelations about bugs in the new building under construction -- lead to unreasonable pressures and unfounded charges in the case of some Marines? The involvement of Sgt. Lonetree, who turned himself in and confessed to numerous acts of espionage, has been fairly clear all along. But unless new evidence is forthcoming, it appears that serious criminal conduct by the Marine guards was not widespread