The espionage trial that begins today at the U.S. District Court in Alexandria centers on the stories of three people for whom the personal tragedy of Vietnam has never ended.

For the American accused of stealing classified cables from the United States Information Agency, for the Vietnamese anti-war activist charged with funneling the documents to Vietnamese officials in Paris, and for the double agent who is the key witness against them, personal affections and alienations are inextricably entwined with the international intrigue that has brought them to the court room.

The American, Ronald L. Humphrey, is the most enigmatic figure in the case.His personal history has been kept a guarded secret by his family and friends. But what little had been revealed suggests a man desperately driven by concern for those he loves.

"I want to say one thing about Humphrey," his codefendant told a reporter recently. "If all Americans loved their families the way he loved his, this would be a better place."

Though he is still married to an American woman named Mary Lou Humphrey, and they are reportedly close friends, the family that Ronald Humphrey loves is Vietnamese: a woman named Chieu Thi Nguyen, whom he calls Kim, and her children by a man who was killed in the war.

Humphrey met them while he was stationed as a civilian adviser in Vietnam between 1968 and 1971.

When he was transferred to the American Cultural Center in Cologne, Germany, he was able to take Kim along, accoding to a letter he wrote in 1975, but at least two of her children had to stay behind with their grandmother because he said he could not afford the $5,000 demanded for exit visas.

In the spring of 1975 Kim's mother died, and when she returned to Saigon as she had done twice before to try to make arrangements for her family, she found herself trapped there by the collapse of the Thieu regime.

For the next three years Humphrey worked relentlessly in Germany and then in the United States (where he was made a watch officer in the USIA communications room) to obtain Kim's release. He wrote to senators and representatives, to the White House and the Red Cross, the Quakers, "even the P. R. G. in Paris," according to one letter, "all either without success or without reply."

In 1976 Humphrey contacted David Truong, who is now about to stand trial with him. Truong was thought by many people in Washington to have lines of communication with the Vietnamese government at a time when there were very few official contacts.

Truong smiled as he recently described his first meeting with a pale, intense Humphrey. "He just appeared," said Truong. "He was so eager to get his wife out!"

Government affidavits maintain that Humphrey began to give Truong information and materials as early as 1976, and many observers suggested these were, in effect, traded for Kim's release from Vietnam.

In a statement made the day he was arrested, however, Humphrey said that Truong had never promised him the information he gave him would help directly to release Kim, except perhaps to improve relations between the United States and Vietnam.

At the same time he was involved with Truong, Humphrey was reportedly working with Swedish diplomats and German novelist Heinrich Boll to free his family.

By July, 1977, Kim was allowed out of Vietnam to visit her sister in Germany, and last Thanksgiving Day she and her children finally arrive in this country.

Two months later the family was separated again as FBI agents arrested Humphrey and jailed him on charges of spying for Hanoi. He is still in jail.

"I'm nobody's agent," David Truong told a group of reporters recently, "and nobody's spy."

Soft-spoken, smiling easily, wearing a dark suit, black-framed glasses and thick-soled American shoes, he looked like the businessman he once said he wanted to be.

A few days before, his friends had raised $250,000 bond to gain his release from jail. His sister and American brother-in-law had supplied about half the money, but the rest had been raised from various charitable sources and individuals attracted as much by his politics as his character.

A biographical press release from The Vietnam Trial Support Committee, established to raise money to defend Truong, made the point that Truong was born 32 years ago on the same day that Ho Chi Minh declared Vietnam's independence from France.

But David Truong did not grow up a revolutionary. He was the son of Southeast Asia's leading Rotarian, Truong Dinh Dzu, a lawyer and politician whose children were born into the elite Francophile society of Saigon. Their playmates were the off-spring of other rich Vietnamese and such influential Americans there as William Colby, then first secretary of the American Embassy.

His father was part of South Vietnam's volatile political world in the early 1960s, and was jailed briefly in 1963 by the Diem regime. But when Truong, then 19, started to get involved with student protests in 1964, he said recently, he was sent to Stanford University in California to keep him out of trouble.

In California, Troung began to see Vietnam as it appeared in the American press and in his father's letters. During his four years in college, both his country and his father were changing profoundly.

"You know," said Truong, explaining how his father was affected, "there's a point when you cannot live in your own country and see what is going on around your house-totally destroyed-eased-by a country you thought could be your friend."

Truong said his father "started out very pro-American." But by 1967, when he ran for the presidency of South Vietnam, Truong Dinh Dzu was a peace candidate-the "White Dove" he was called. He came in second to Nguyen Van Thieu, and Thieu had him jailed for the next five years.

Truong, who said he had just been drifting through college up to that point,set to work collecting help for his father and pressing for an end to the war. He soon found himself the focus of attention.

By August 1968, just before the tumultuous Democratic convention in Chicago, the late columnist Drew Pearson wrote: "Of the several million youngsters in this country urging peace in Vietnam, probably the most effective is David Truong."

With Pearson's help, Truong said, he was introduced to some of the most powerful people on Capitol Hill, where several congressional aides remember him as a valuable source of information about what was "really" happening in South Vietnam.

Much of his information, Truong said, came from his father, who "was pretty much in touch with all sides during the war," including the Vietcong and the North.

"A Vietnamese who has any feeling for his country," said Truong, "cannot help but feel we all struggle together. You cannot help but respect your compatriots who have sacrificed more than anybody to keep your country independent and free."

Truong said his father was given the chance to leave Vietnam as Hanoi's troops were rolling toward Saigon, but he refused."He considers himself a patriot and he would never leave his country."

By April 1975, however, Truong had been away from Vietnam for 11 years, and he did not return. Instead, he started a Washington organization called the Vietnamese-American Reconciliation Center.

He found, however, that interest in his country had all but evaporated in Congress. His friends on Capitol Hill say they talked mostly about potential trade between the two countries.

With the end of the war, Truong told a reporter, he began to make plans for himself. "My dream," he said, "has been to stablize my personal living; maybe, time permitting, to set up the first multinational [corporation] owned by Vietnamese."

With his friends in the United States and his family's contacts in Vietnam, he would have been in a good position to broker trade between the two countries, but it never developed.

For the last three years Truong has supported himself with various jobs and tried to work with the Vietnamese refugee community, but there too, many say he found a cold reception among the vehemently anticommunist majority.

Several said they found it objectionable that he advocated aid for the communist regime in Vietnam. He made no secret of his position.

"Now that the war is over," he told a reporter, "I think everyone has to send what he can." He mentioned books about oil, about medicine, about agriculture.

It was a box full of just such books that FBI reports say Truong gave to double agent Dung Krall outside a Dupont Circle cafeteria (the exact one is unknown) on April 19, 1977, to be taken to Vietnamese officials in Paris.

But the FBI said he gave her another package as well, which they subsequently opened. Inside were more than 45 classified State Department documents.

It was on the basis of these, and other documents intercepted later, that Truong was charged with being a spy for Vietnam.

Close friends of Dung Krall, who say they had no idea she was working for the CIA and the FBI, think of her principally as a good wife to her American husband John Krall-a former pilot now assigned to Naval Intelligence Command-and a good mother to her eight-year-old son, Lance.

As for politics Dung Krall's friends remember she and her mother and sisters always said they hated the communists. "When we talked about her father," one told a reporter, "we say that he died."

But Krall's father is very much alive. During the late '60s and early '70s he was the Vietcong ambassador to the Soviet Union, Dang Quang Minh.

Since her function as a double agent was made public last January, the United States government has kept Krall's whereabouts and background a carefully protected secret. Her only appearance so far has been a single day of testimony during preliminary hearings on the case.

But close friends of hers and her family have made it possible to piece together some of her background.

Her father went to Hanoi in 1954, leaving Krall's mother to raise her, her four sisters and brother alone in South Vietnam.

The brother, Tran Van Van, became the man of the family, and people who knew him in Saigon remember him as a bright, attractive and popular young man, a self-taught guitarist and painter.

By the early 1970s, he was part of South Vietnam's army, sent to the United States for training as a helicopter pilot. Just before he was to return home, he was killed while flying near Savannah. Ga.

It was a blow to the family from which it never fully recovered, friends say.

During the '60s, three of the sisters, including Krall, had moved to the United States. In the spring of 1975, just a week before Saigon fell, their mother and two youngest sisters came to this country to join them.

To add to the trauma of their dislocation, soon after they arrived, they heard that the Provisional Revolutionary government of South Vietnam had bulldozed the cemetery in Saigon where their brother had been buried to use for other purposes. They also heard that their father, Dang Quang Minh, by then a member of the Provisional Revolutionary Government politburo, had made no effort to stop the desecration.

Though she did not mention the incident in court, it was at about that time that Krall said she decided to work for the Central Intelligence Agency.

Her friends remember that she traveled a great deal-to France, to Denmark, and to Japan-in recent years.

In August 1975, according to sources close to the investigation, she reportedly went to Hiroshima, Japan, to meet with her father there. It would have been the first time, friends believe, that she had seen him in 20 years.

Soon after that meeting according to Krall's testimony in court, she began receiving letters from Vietnamese officials in Paris, suggesting that she contact certain people in the United States. One person she met was David Truong, for whom, she said, she began to carry packages to Vietnamese officials.

Government documents say that she also came to know Vietnam's ambassador to the United Nations (since expelled from the United States) Dinh Ba Thi, and a Vietnamese embassy official in Paris named Phan Thanh Nam.

Both were named as unindicted co-conspirators on the day that Truong and Humphrey were arrested. Both were former officials of South Vietnam's Provisional Revolutionary Government who would have known Krall's father, State Department sources said.

Whatever other motives Krall may have had in working for the United States government, she was paid for her work. Government documents reveal that she received $20,200 between June 1976 and April 1977, and continues to receive $1,200 a month. All her travel as an agent was paid for, and the government has spent at least $11,800 to relocate and protect her and her family.