Marine PFC Robert Garwood left Vietnam for the first time in 13 1/2 years yesterday to face Marine Corps charges of desertion and working with the Vietcong to get other American troops to throw down their arms.

Garwood, 33, refused to talk with reporters when he landed in Bangkok. In a television interview from Hanoi Wednesday night, however, he denied betraying the United States while conceding that he opposed America's Indochina policy.

"I betrayed the involvement of the U.S. government in Vietnam because I did not carry out the policy of the United States government to kill and to demolish these people and this land," he told an NBC interviewer.

Garwood, who was 19 when he disappeared while driving alone in a jeep near Da Nang, South Vietnam, on Sept. 28, 1965, said he decided to return "because I am an American. I'm not a Vietnamese."

But he spoke during the interview with a decidedly Vietnamese accent, which was noted by his brother Donald E. Garwood of Indianapolis.

He had a distinctly unmilitary appearance when he alighted from the weekly Air France flight to Bangkok from Ho Chi Minh City, formerly Saigon. He was dressed in jeans and a checked shirt, and wore a Thai orchid corsage on his chest and a wide silver bracelet on his right hand. His hair was thinning on top, but he had wide pork chop sideburns.

Garwood was met by a U.S. consul and a Marine Corps representative, who escorted him to a military plane where he was informed of the military charges against him and told of his rights. A lawyer assigned him by the Marines was on the plane, which flew him to a military hospital on Okinawa.

He has remained on the Marine Corps roster since his disappearance.

The five charges filed against him at Marine headquarters in Washington are the first step in a complicated legal process that could lead to a court martial. If convicted of all charges by a court martial, Garwood could face the death penalty, but Marine officials said no members of the naval services have been executed since the turn of the century.

The charges are the formalization of accusations hanging over Garwood's head since the end of the U.S. involvement in Vietnam, Marine officials explained.

They include soliciting American troops to refuse to fight, desertion in time of war, unlawful communication with the enemy, misbehaving as a prisoner of war and attempting to cause insubordination, disloyalty and refusal of duty among his fellow POWs.

Other Marines have charged that Garwood carried weapons on Vietnamese patrols against American forces and took part in brutal questioning of American prisoners.

Garwood, a high school dropout from the town of Greensburg, Ind., near Indianapolis, said on the NBC interview in Hanoi that he warned "my fellow prisoners" against trying to escape because "escaping is suicide." *tHe said that he did not collaborate with the Vietnamese.

"I did everything of my own free will. I was in coordination with the peace movement in the United States against the involvement of the United States government in Vietnam."

Garwood said any evidence against him "is evidence against the whole peace movement, not only in the United States but the whole world."

He said the peace movement changed his views on the U.S. involvement in Vietnam. Before that, he said, "I didn't know anything about this war, you know. I didn't know why I was here."

Radio Hanoi gave no details of Garwood's activities during the past 13 1/2 years in Vietnam except to say that he "changed sides" during the fighting.

"Before leaving," Radio Hanoi continued, "Garwood expressed his gratitude for the good treatment given to him during his stay in Vietnam."

His father, Jack Garwood, said in Greensburg yesterday that Garwood "should be spanked" but that the formal mArine Corps charges against him should be dropped.

While he was in Vietnam, Garwood accumulated $146,000 in back pay which is awaiting him at the Pentagon. But that money could be seized if he is convicted by a court martial.

Garwood had dropped out of sight until last month, when he asked a Western traveler in Hanoi -- reportedly a Finnish diplomat -- to tell the U.S. government that he wanted to return home.

He was still listed as a prisoner of war by the Defense Department even though he was reported to have chosen to stay behind when most American prisoners were repatriated in 1973.