W.S. Merwin, whose austere lyricism in poems about the fragility of the natural world and the horrors of the Vietnam War earned him two Pulitzer Prizes and made him one of the preeminent English-language poets of the past five decades, died March 15 at his home in Haiku, Hawaii, on the island of Maui. He was 91.

The death was confirmed by Sonnet Coggins, executive director of the Merwin Conservancy. No specific cause was cited.

Along with John Ashbery, his elder by two months, Mr. Merwin was one of the defining American poets of his generation, a prodigious and prolific talent who wrote two dozen books of poetry as well as story collections, memoirs, plays and acclaimed translations.

While questioning American exceptionalism, he drew comparisons to Walt Whitman; in his descriptions of death and absence, he was likened to Samuel Beckett. He visited a mental ward to learn from Ezra Pound, lived with Robert Graves on the island of Majorca, drew the affection of Sylvia Plath, and was encouraged by Robert Lowell and W.H. Auden.

Yet he also developed a voice all his own, pulling from his Presbyterian upbringing and years spent studying European literary traditions to craft works that were insistently plain-spoken while also suggestive of spiritual mystery.

In poems such as “Low Fields and Light” (1955), among the first of about 200 pieces he published in the New Yorker, he described a vision of nature that was simultaneously otherworldly and anchored in vivid imagery:

I think it is in Virginia, that place

That lies across the eye of my mind now

Like a gray blade set to the moon’s roundness,

Like a plain of glass touching all there is. …

My father never plowed there, nor my mother …

But you would think the fields were something

To me, so long I stare out, looking

For their shapes or shadows through the matted gleam, seeing

Neither what is nor what was, but the flat light rising.

Mr. Merwin spent much of his career looking — and forcing readers to look with him — at a world that seemed to be falling apart. “The human race,” he once said, gesturing toward mass extinction and environmental catastrophe, “is a terrible mistake.”

He came to prominence with “The Lice,” a bleak 1967 collection that addressed the destruction of nature and the escalating war in Vietnam. “Nights,” he wrote in “The Asians Dying,” “disappear like bruises”; the seasons, unobserved, called out like “paper bells.”

His follow-up, the 1970 volume “The Carrier of Ladders,” was no less dark, and perhaps more pessimistic than the work of T.S. Eliot, the literary critic Helen Vendler wrote in the New York Times. The language of “Ladders” and a companion book of prose, “The Miner’s Pale Children,” was so dark as to border on the excessive, if not simply the obsessive.

“The Merwin dictionary,” she wrote, “has nouns of ill-omen (pain, grief, fear, pallor, extinction), obsessive objects (gloves, hands, clocks, watches, bandages, shrouds and eyes), exhausted adjectives (hollow, empty, faint, deaf, blind, blank, frozen, lost, broken, hungry, dead), and constellations of negation (speechless, colorless, nameless, windless, unlighted, unseen, unmoved, unborn). Is it ill-will in a reader to want to force-feed these pale children till they, when cut, will bleed?”

Despite its grimness, “Ladders” was awarded the Pulitzer Prize in 1971. Mr. Merwin, a pacifist who had recently begun studying Buddhism in earnest, was not content to accept the honor quietly. He responded by penning a short, scathing public letter to the New York Review of Books.

“After years of the news from Southeast Asia, and the commentary from Washington,” he wrote, “I am too conscious of being an American to accept public congratulation with good grace, or to welcome it except as an occasion for expressing openly a shame which many Americans feel, day after day, helplessly and in silence.”

Mr. Merwin went on to say that he planned to donate the prize money to “the Draft Resistance” movement and to Alan Blanchard, “a painter who was blinded by a police weapon” during a demonstration in Berkeley, Calif.

Though the letter began by expressing gratitude to the Pulitzer judges, it irked old-guard poets such as Auden, who wrote a response one month later chastising Mr. Merwin for what Auden described as an “ill-judged gesture” bordering on “a personal publicity stunt.”

The Pulitzer thrust Mr. Merwin fully into public view, but he was helped along, perhaps, by his handsome, square-jawed appearance, which led the New Yorker poetry editor Howard Moss to declare, “Nobody has a right to be that good-looking.”

In a sign of his longevity, Mr. Merwin earned Yale University’s Bollingen Prize — a career capstone for most poets — in 1979, decades before he produced some of his most acclaimed work, including the National Book Award-winning “Migration: New and Selected Poems” (2005) and “The Shadow of Sirius,” an autobiographical volume that earned him his second Pulitzer in 2009.

Mr. Merwin also won what is now the Wallace Stevens Award, a $100,000 prize given by the Academy of American Poets, and was twice honored by the Library of Congress as a poet laureate, most recently from 2010 to 2011.

Learning the language

William Stanley Merwin was born in New York City on Sept. 30, 1927, and raised in Union City, N.J., and Scranton, Pa. His father was a Presbyterian minister for whom a 5-year-old Mr. Merwin produced church hymns, his first written works; Mr. Merwin described him as “a bully” prone to committing acts of violence at home. His mother was an orphan who had lost her brother and then her first child.

Mr. Merwin, who went by William, began seriously reading poetry — and then, “incessantly, and with abiding desperation,” attempting to write it — while at Princeton University. He found mentors in the confessional poet John Berryman and literary critic R.P. Blackmur and once made a pilgrimage to St. Elizabeths mental hospital in Washington to see Pound, the fascist-friendly poet who had evaded accusations of treason by pleading insanity.

To become a poet, Pound said, write at least 75 lines a day, and to find material, he added, translate works in other languages. “Translation,” Mr. Merwin recalled him saying, “is your way of learning your own language.”

Mr. Merwin took his advice. After graduating in 1948 with a bachelor’s degree in English, he married a Princeton secretary, Dorothy Ferry, and struck out for Europe. Immersing himself in the literary traditions of Spain, France and Italy, of ancient Greece and Rome, he translated classic works and worked as a tutor, including for the son of Graves, the novelist and poet.

Mr. Merwin’s translations were as widely praised as they were wide-ranging, encompassing Sanskrit love poems, Greek tragedies, the French medieval epic “The Song of Roland,” the Chilean poet Pablo Neruda and the “Purgatorio” of Dante.

As Pound suggested, they also provided grist for his own work, beginning with the 1952 collection “A Mask for Janus.” The book drew from ancient myth — a key touchstone for Graves as well — and was selected by Auden for the Yale Younger Poets Prize . Mr. Merwin later judged the prize himself and shocked some in the poetry community when, in 1997, he declared that none of the 700 entrants was worthy of America’s oldest annual literary honor. “Too uneven,” he said.

A rain forest on Maui

Initially, Mr. Merwin’s poetry hewed closely to classical forms. Through the years, however, his verse became looser and more free-flowing, with capitalization and punctuation eventually disappearing altogether.

In part, it was the result of a change in lifestyle.

Mr. Merwin’s first marriage ended in divorce after Graves introduced him to Dido Milroy, an Englishwoman 15 years his senior. By the late 1960s, that relationship had also fractured, and Mr. Merwin began delving into Buddhism.

He was a regular at the Naropa Institute in Boulder, Colo., a favorite retreat of Beat poet Allen Ginsberg, but — following a drunken scuffle known as “the Great Naropa Poetry Wars” — he eventually moved to Hawaii in 1976, studying with Zen master Robert Aitken.

Mr. Merwin settled atop a dormant volcano in Maui, near the poetically named town of Haiku, and set to work transforming a failed 18-acre pineapple plantation into a nearly pristine plot of rain forest. For the rest of his life, he split his time among gardening, meditation and poetry, composing works that were increasingly concerned with conservation and — at least by Mr. Merwin’s standards — increasingly optimistic.

“I want to tell what the forests/ were like,” he wrote in “Witness,” an 18-word poem from “The Rain in the Trees” (1988). “I will have to speak/ in a forgotten language.”

Mr. Merwin’s later works included “Travels,” which fictionalized historical figures from the poet Arthur Rimbaud to the botanist David Douglas, and “The Folding Cliffs” (1998), an epic poem about late 19th-century Hawaii that poet Ted Hughes hailed as “a truly original masterpiece.” His most recent collection of new poems, “Garden Time,” was published in 2016.

He married Paula Schwartz, a children’s book editor, in 1983. They established the Merwin Conservancy to preserve their home as a writers’ retreat and to conserve the surrounding forest, which is now home to more than 400 species of palm trees.

When Mr. Merwin developed macular degeneration in recent years, she served as his literary assistant, helping him read and write until her death in 2017. A complete list of survivors was not immediately available.

For all his acclaim, Mr. Merwin sometimes expressed uncertainty about the quality of his work. In his poem “Berryman,” he recalled a harrowing conversation with his mentor at Princeton:

I had hardly begun to read

I asked how can you ever be sure

that what you write is really

any good at all and he said you can’t

you can’t you can never be sure

you die without knowing

whether anything you wrote was any good

if you have to be sure don’t write