Years ago he was General Cao Van Vien, the man who ran the Army of the Republic of South Vietnam during its final traumatic 11 years, defended Saigon during the Tet Offensive and stood on countless reviewing stands with the highest American military brass.

This week Vien stood, unnoticed, beside his wife in a crowded Alexandria courtroom, right hand raised as they and 63 others became naturalized citizens of the country that once poured lives and millions of dollars into trying to save their homeland.

"He was absolutely a key figure" in the years of American involvement in Vietnam, says Robert Komer of Alexandria, the former chief U.S. adviser on pacification of the Vietnamese countryside. "He was energetic, hard-working, very low-profile and self-effacing."

Since he moved here his manner has remained the same, so much so that Komer was unaware until recently that Vien -- who once conferred constantly with the likes of Creighton Abrams and William Westmoreland, the senior American commanders in Vietnam -- was living only a few miles away in suburban Falls Church.

Vien left Monday's ceremony in U.S. District Court holding his citizenship certificate and politely but firmly declining to be interviewed. "No comment," he said. "I still have many enemies."

"If you lived through what Vien must have lived through, especially after the Americans left Vietnam ," says Komer, "it's bound to have a traumatic effect."

"He often said, 'We did the very best we could, we worked hard at it,' " says retired Gen. William E. Potts, who knew Vien in Vietnam. "He said, 'We shared in the victories and successes and we must share in the things that went wrong.' "

Potts, now an employe of General Research Corp. in McLean, has since overseen the writing of a $1 million, three-year history project for the Army in which former Indochinese military officers, including Vien, were paid up to $1,500 a month to record their views of the Vietnam War.

Lt. Col. Adrian Traas of the Army Center for Military History said Vien's contribution, "The Final Collapse," will be published in a few months. It will be issued in book form throughout the Army and be available to the public through the Government Printing Office, Traas said.

Vien was wounded in combat as a colonel in 1964, reportedly a time when few senior Vietnamese officers saw action on the battlefield. The same year he was made chief of the Vietnamese joint general staff. For the next decade he commanded the country's armed forces, leaving in 1975 as the enemy was entering Saigon.

On Monday as District Judge Albert V. Bryan Jr. congratulated the group of new Americans, including about a dozen Vietnamese. "Like an adopted child who learns he has been chosen by his new parents," Bryan said, "this country delights in being chosen by you for citizenship."