The poet Carolyn Forché begins her gripping new memoir with a commission from an imperious, mysterious stranger. Leonel Gómez, the cousin of a friend, has driven three days, all the way from El Salvador to Southern California, to recruit her as a witness to his country’s woes. It is the late 1970s, and El Salvador, he insists, is on the brink of a civil war. Describing the situation as “the beginning of another Vietnam,” he calls up his listener’s memories of gruesome combat stories her former husband told. During the next three days, he gives Forché a crash course in Salvadoran history, economics and politics. He exhorts her to visit the country with eyes wide open in order to tell the people of the United States — which plays a decisive role in his country’s plight — what she has seen.

The summons is almost biblical. When Moses is ordered to confront Pharaoh, he tries for seven days to evade the divine imperative. And Forché, a one-book poet in her 20s, hesitates. Though Leonel has sought her out because of her literary skills, she asks: “What does this have to do with poetry?” Challenging her notion of vocation, Leonel demands: “Are you going to write poetry about yourself for the rest of your life?” After considerable soul-searching, Forché puts aside the student papers she’s grading and flies to Central America. A Guggenheim fellowship makes it financially feasible for her to pack up and head off by herself for a prolonged stay in a foreign land. “What You Have Heard Is True” recounts how and why its author began to write the kind of poetry she published in her 1981 book “The Country Between Us,” what she calls “the poetry of witness.”

Though Leonel is a coffee farmer, champion marksman, motorcycle racer, painter, inventor and scholar, his true identity remains elusive. Is he a revolutionary? A government informer? A freelance opportunist? An agent of the CIA? He is adept at orchestrating what he calls a “symphony of illusion,” a strategy to put potential assailants off-guard by keeping them guessing about just who he is. Leonel refuses to talk about himself and disappears for days at a time. But he is somehow able to gain Forché access to a wide range of Salvadoran society — leftist guerrillas, campesinos, clergy, medical volunteers, businessmen, indigenous people, military officers, enlisted men, Salvadoran poets, death squad members, aid workers, even the U.S. ambassador. Her best-known poem, “The Colonel,” will draw on a visit with a uniformed butcher who flaunts the ears he collects from his victims.

With 37-year-old Leonel as her Virgil, Forché immerses herself in the inferno that El Salvador has become — with the complicity of the U.S. government and American corporations. She sees the abject poverty of peasants forced to work themselves into early graves and observes the operation of a death squad on the streets of the capital. She sees corpses and parts of corpses scattered throughout the countryside. Visiting a political prison, she sneaks a peek into a room where men are confined to coffinlike boxes for months. “Tell them on the outside,” a prisoner pleads, “tell them.” Through it all echoes Leonel’s injunction: “You have to be able to see the world as it is, to see how it is put together, and you have to be able to say what you see.”

The young American woman’s presence in El Salvador on the eve of full-scale conflict places her in mortal danger. Avoiding the fate of the desaparecidos, those who are abducted and never heard from again, she is nevertheless shot at, as well as hospitalized with dengue fever and dysentery. She conducts an interview with the saintly Archbishop Oscar Romero hours before he is assassinated while celebrating Mass. Forché is spirited back to the United States just as the war that Leonel predicted erupts. It will last 12 years and claim more than 75,000 lives. Its failure to resolve fundamental social problems is apparent in the current influx of desperate Salvadorans to the borders of this country.

One might expect a poet’s prose to be florid, or at least highly metaphorical. But Forché honors her responsibility to the Salvadoran calamity by writing sparely and precisely. On occasion, recalling the verdant fields and starry nights of Central America, she can wax lyrical. More often, the poet’s voice is manifest in a luminous sentence such as: “The weather was white with the coming rains.” Much of the book consists of extended conversations between Forché and the varied people she meets. And though it is highly improbable that she could remember everything that was said verbatim, what appears in the book seems plausible.

“I could just as well write my poetry from the quiet of my own study,” Forché writes, “but I had known since childhood that human suffering demanded a response, everywhere and always.” A portrait of the artist as political and poetic ingénue, “What You Have Heard Is True” is just such a response, a riveting account of how she made good on that conviction. It bears eloquent witness to injustice and atrocity and to how observing them shaped a fearless poet.

Steven G. Kellman’s latest book is “The Restless Ilan Stavans: Outsider on the Inside” (University of Pittsburgh).

What You Have Heard Is True

A Memoir of Witness and Resistance

By Carolyn Forché

Penguin, 400 pp, $28