Kim Jones was back from active duty in Vietnam eight years when, on his 32nd birthday, he turned himself into, as he put it, “a walking sculpture that’s eighteen miles long.”

He put on combat boots, pulled a nylon stocking over his face and donned a makeshift crown of foam rubber and aviary wire. He slathered his body in mud and strapped to his back a weird, latticelike construction of sticks tied with rope. He then walked westward along Wilshire Boulevard from downtown Los Angeles toward the Pacific Ocean. Toward Vietnam.

Before going to Vietnam, Jones had been an art student. After it . . . what?

The cruel, unanswerable question of how art should respond to war is at the heart of “Artists Respond: American Art and the Vietnam War, 1965-1975,” a must-see show at the Smithsonian American Art Museum. It’s an exhibition of miscellaneous work made by a diverse group of artists during the peak years of a war that changed everything — including art.

It’s the first time the Vietnam War has been addressed on this scale by an art museum. Organized by Melissa Ho, it pulsates with anguish from first to last. And it reminds us that though Saigon fell more than 40 years ago, that anguish is still with us. Today’s polarized politics — from the culture wars to congressional gridlock — are shot through with the aftershocks of Vietnam (just think of the recent tweets exchanged by President Trump and John McCain’s daughter Meghan McCain), as are the memories, family histories and inner lives of millions.

“Sweat like pigs work like dogs live like rats red dust covered everything.” That’s how Jones described his time in Vietnam. For bystanders, “Mudman” — the persona he created for performances that led to “Wilshire Boulevard Walk” — was frightening, dissonant, uncomfortable. But it was still more uncomfortable for him.

“Wilshire Boulevard Walk” wasn’t intended as a metaphor. It wasn’t even explicitly about Vietnam — although, according to Jones, other veterans got it, and several approached Mudman saying, “Yeah, I know what it was like.” Instead, the performance expressed Jones’s personal struggle and the wider crisis of American society.

The shock of Vietnam made conventional art forms such as painting and sculpture look inadequate. Its reverberations inspired a rapid expansion of the possible forms art could take and a search for new audiences. Public performances, video, installations, land art and agitprop all flourished during the war.

Vietnam also opened up avant-garde art to previously neglected voices, including women, African Americans, Latinos and Asian Americans. There has been a long delay in recognizing this, and part of the triumph of “Artists Respond” is that it shows how many artists from marginalized groups gave their unique responses to the war urgent and potent expression.

African American and Latino populations bore a disproportionate load under a draft system that was patently unfair. Black and Latino soldiers returned from Vietnam in the late 1960s and early ’70s to a society still riven by discrimination. It’s no surprise, then, that some of the most explicitly activist art of the period was made by African Americans and Latinos, including Faith Ringgold, David Hammons and Malaquias Montoya.

These artists didn’t have to be told that the war was connected to the fight for civil rights. They felt it intimately, every day. But their responses were artistic, not mere sloganeering. Hammons’s “America the Beautiful” shows a black body and face draped in an American flag. He made the figure by pressing his greased body to paper, then sprinkling the paper with pigment. The resulting image haunts, its ghostliness keyed to the ambivalent predicament of young black lives even under a banner of patriotism.

The final years of the Vietnam War — the early 1970s — coincided with feminism’s second wave. Artists such as Carolee Schneemann (who died this month), Yayoi Kusama, Judy Chicago and Corita Kent rode and shaped that wave, articulating powerful critiques of the patriarchal forces that create and feed off war.

A lot of antiwar performance art emphasized the vulnerability of the body. Yoko Ono’s “Cut Piece” was staged five times between 1964 and 1966, including at New York’s Carnegie Hall in 1965. Ono sat on a floor, a pair of scissors in front of her, and invited audience members to come forward one by one and cut off pieces of her clothing.

The performance, at heart, feels feminist. But in a year that saw U.S. troop levels in Vietnam ratcheted up from 23,000 to 184,300, it was also easy to read as a comment on the war. Some commentators erroneously identified Ono (who is Japanese) as representing Vietnam. But “Cut Piece,” like Jones’s “Wilshire Boulevard Walk,” was less a metaphor than a form of art-as-psych-experiment: Given the license, how much will you cut off? And how long will you watch a situation that is getting more and more disturbing before you intervene?

The wider situation got more disturbing by the day, as bodies came home, and Americans watched it all on television. They saw monks setting themselves on fire, close-range executions and naked, running children burned by napalm. A constant stream of less notorious but scarcely less horrific images compounded their dismay. The propaganda and these images didn’t match up.

Martha Rosler, who was trained as an abstract painter, turned to making photomontages that highlighted the contradictions. She jammed horrific images from Vietnam into magazine photos of affluent American interiors, reminding people that, as a brochure accompanying one of her exhibitions put it, “the war is always home.”

Artist Chris Burden responded to the anguish of seeing, as he said, “a lot of people being shot on TV every night, in Vietnam, guys my age” with a 1971 performance that has become notorious and still hurts the brain to contemplate. He had a marksman aim a rifle at him and shoot him in the arm.

If you find it obscene that an artist should do such a thing while “guys his age” were being shot at and killed in chaotic circumstances far from home, so do I. But Burden was picking up on a level of irrationality that no longer felt exceptional. It was ambient.

When you think of how much was going on in the world, it can seem obscene that the most critically acclaimed American artists of the 1960s were spray-painting pretty patterns on giant canvases (Kenneth Noland), painting enlarged comic strips (Roy Lichtenstein) and fabricating metal boxes (Donald Judd).

Many of them, to be fair, were simply trying not to claim too much for art. The catastrophe of World War II had taught them that art’s place in the world was modest alongside the realities of war, genocide and nuclear destruction. They had watched fascists and communists try to convert art into an instrument of political power. They wanted to protect it from the task of mass persuasion.

That didn’t mean they didn’t hate the war or that they failed to oppose it. Ad Reinhardt, for instance, kept on painting his nuanced minimalist abstractions even as he became active in the protest movement. For an antiwar protest group, he made a ­postcard-style work illustrating his position. Addressed to “War Chief, Washington, D.C.,” it says “no war,” “no draft” and “no fear” on one side and “no art of war,” “no art about war” and “no art as war” on the other.

But the pressure they felt to keep art and activism apart became close to unbearable for many of the most acclaimed practitioners of abstraction and minimalism. Some, including Judd, Dan Flavin and Carl Andre, moonlighted as makers of explicitly antiwar art. Others, like Rosler, completely changed the kind of art they made.

Philip Guston had become famous for his “abstract impressionist” paintings in quivering pinks and greys. But, he said, “I was feeling schizophrenic. The war, what was happening to America, the brutality of the world. What kind of man am I, sitting at home reading magazines, going into a frustrated fury about everything — and then going into my studio to adjust a red to a blue?”

In 1970, he switched from abstraction to a clunky kind of figurative painting, with legible, recurring forms (a lightbulb, Ku Klux Klan hoods, shoes, bricks, cigarettes) and an overall vision that felt mired in crisis. Guston is represented here by his most explicit expression of contempt for corrupt power: a blistering caricature of President Richard M. Nixon.

Some painters went further. “I wanted to make the ugliest paintings I could,” said Judith Bernstein. “I wanted them to be as ugly and horrifying as the war was.” In paintings such as “A Soldier’s Christmas,” which drew on the violent and sexual blurtings of bathroom graffiti, she made good on her wish.

Elsewhere, painters kicked against the coolness of pop art and the mute elegance of minimalism by making work that was angry, hectic and hot: Jim Nutt and Peter Saul, members of the Chicago-based group the Hairy Who, represented torture and debauchery with paintings that were psychedelically vibrant even as they delivered up what Saul called a “cold shower” of “bad conscience.”

All kinds of artists were trying to find forms to contain the war, to give it meaning. The source of their anguish was that they couldn’t: There was no proportionate response. The disaster was too large; their art — art itself — too small.

“Paintings don’t change wars,” as the passionately political painter Leon Golub said in 1967. “They show feelings about wars.”

Are feelings enough? If you compare the art in “Artists Respond” to the magnitude of the war, it can seem paltry. That does not mean it wasn’t poignant, courageous, estimable. It was just no match for the bigger forces at work.

Being no match for bigger, catastrophic forces is the very definition of tragedy. And if tragedy teaches us anything, it is humility.

That’s why, for me, the abiding image from this show is still the humble wooden lattice Kim Jones wore on his back as Mudman. It leans against a wall, like a prop from a Samuel Beckett play. Next to it are his mud-covered combat boots (they can’t help but put you in mind of van Gogh’s shoes). “Wilshire Boulevard Walk” was about more than the Vietnam War. It was about homelessness, humility and heartbreak.

Artists Respond: American Art and the Vietnam War, 1965-1975 Through Aug. 18 at the Smithsonian American Art Museum, Eighth and F streets NW.