Marine Sgt. Clayton J. Lonetree betrayed his country because he loved a Soviet woman and wanted to avenge the government's treatment of American Indians, a military prosecutor said yesterday.

"In his statements, he admits what is obvious, that he was helping the Soviet Union," Marine Maj. David L. Beck said in opening arguments at Lonetree's court-martial at Quantico Marine Corps base.

"The accused gave away documents and vital information important to our intelligence and our national defense interests," Beck said after beginning the government's case by reciting Lonetree's oath to defend the United States.

Lonetree, the first Marine to be tried for espionage, is accused of revealing the identities of Central Intelligence Agency officials and U.S. Embassy floor plans to the Soviet KGB secret police during his stint as an embassy guard in Moscow. If convicted, Lonetree could be sentenced to life in prison.

Defense attorney Michael Stuhff described Lonetree, 25, of St. Paul, Minn., as a patriot who did not give the Soviets anything of value. He said Lonetree wanted to become a double agent.

Lonetree's mistake, Stuhff said, was trying to take on the KGB singlehandedly, dealing with a man named Sasha, whom prosecutors say is a KGB agent.

"Sgt. Lonetree did not compromise his oath," Stuhff said in his opening statement. "He made a mistake. He tried to take on the KGB." Stuhff questioned whether Sasha was a KGB agent.

Beck said Lonetree became involved in espionage after beginning a clandestine love affair with a woman named Violeta, who worked as an interpreter in the embassy. He also said Lonetree, an American Indian, may have been motivated to spy by a desire for revenge against the federal government's treatment of his people.

The defense acknowledged the love affair but said spying was not involved.

Stuhff said the Soviet woman led Lonetree to believe that she was scared of Sasha, so he tried to go after Sasha himself.

"He tried to set up a blackmail attempt against Sasha in Vienna," Stuhff said. "It wasn't successful, but he tried to do it."

The defense contended that Beck brought up dual motives of sex and revenge because there was no real motive in a case it says is based largely on misguided official theories of "a nest of spies" among Marine embassy guards.

Yesterday's proceedings began with defense attorney William Kunstler telling the judge, Navy Capt. Philip F. Roberts, that the government has not followed through on its promise to take Lonetree out of solitary confinement.