His childhood ended in June 1941, when the Luftwaffe bombed Belarus. “We left the city on foot,” recalls Volodia Korshuk, who was 7 when Germany attacked his hometown of Brest, on the border with Poland. “Before my eyes a stone house ahead of us fell to pieces and a telephone flew out of a window. In the middle of the street stood a bed; on it lay a dead little girl under a blanket.”

Mr. Korshuk is among the 100 survivors interviewed by Svetlana Alexievich for “Last Witnesses: An Oral History of the Children of World War II.” It’s a technique Ms. Alexievich, a winner of the Nobel Prize in literature, has used to great effect in her previous books, bringing to life the dramatic events of Soviet history through ordinary people’s voices. “In my books these people tell their own, little histories,” Ms. Alexievich has said, “and big history is told along the way.” In Ms. Alexievich’s books, such as “Zinky Boys” (1989), about the Soviet-Afghan war, and “Voices From Chernobyl” (1997), eyewitnesses speak alone; the author is silent.

In “The Unwomanly Face of War,” Ms. Alexievich captured the experiences of Soviet women who fought on the Eastern Front in World War II. Completed in 1983, the book was suppressed by Soviet censors until 1985, appearing simultaneously, in the Soviet Union, with its sequel, “Last Witnesses.” (The English translation of “The Unwomanly Face of War” came out in 2017.)

In “Last Witnesses,” each account takes up only a page or two and forms a larger story about ethnic cleansing in Belarus, implemented as part of the German Generalplan Ost. Some of the survivors in the book had witnessed their parents being killed in front of them. Never would they learn more about the war than they did in those moments; the trauma would prevent them from sharing their memories for years to come. “I saw what shouldn’t be seen,” says Yura Karpovich, who was 8 when the Nazis ravaged his village. The memories have scarred him for life: “I grew up with this . . . I grew up gloomy and mistrustful.” Valya Brinskaya, who was 12 when the war began, expresses the idea of the book’s title: “We are the last witnesses. Our time is ending. We must speak.”

One out of every four Belarusians––in a nation of nine million––perished during the war, from starvation, bullets or slave labor. Countless children were orphaned. The Nazis used Belarusian children as human shields, blood donors and slave laborers; they executed many of them alongside the adults. “I really didn’t want to die,” remembers Sasha Solianin, who was 14 when he and two Soviet prisoners of war were taken to be shot. “I especially didn’t want to die at dawn.” The older men attacked the guards and told Sasha to run; he alone survived.