Mr. Thoang’s mother also had a Vietminh husband who went north in 1954, when Mr. Thoang was 1 year old. In 1957 his mother became a concubine of the chief of the commune and gave birth to sons in 1960 and 1962. On a night in 1963, she and the chief were arrested and murdered by the local Vietcong.

At age 10, Mr. Thoang had to take care of his two half brothers in a government strategic hamlet, going alone to the jungle to work as a charcoal burner.

“Our life was so miserable,” Mr. Thoang said in an interview in his house at Cam Chinh, in central Vietnam. “We starved all day. The authorities allowed us to bring only a small beer can of rice out of the hamlet because they were afraid that we would provide rice to Vietcong in the jungle. I had to eat wild bananas to survive.”

Every three to six months local administrators and security officers interrogated and tortured the families of Vietcong soldiers to ferret out Communists and sympathizers, Mr. Thoang said. But almost all families in disputed lands were divided in loyalty. Many parents faced the heartbreak of having their children face off and die on different side of battlefields.

“My grandfather’s four children, including three sons and a daughter, were Vietcong fighters,” said Vo Quyet, who lived as a child in Trieu Phong, a strip of coast north of Hue and the site of bloody battles in the summer of 1972. “Meanwhile, my grandfather’s brother’s four sons were ARVN soldiers” — South Vietnamese troops. “All of them were killed in action but my aunt.”

“Our people were forced to choose a side,” he said. “We did not have rights; we did not have choices; we could not decide our destinies.”

Dao Cong Kiet, 75, who lived in Trung Hai, a commune near the Demilitarized Zone, said his family, too, had to walk a fine line. “We needed to treat both sides well,” he said. When American and South Vietnamese troops came, “I invited them to eat bananas or drink tea.”