So far, Ken Burns and Lynn Novick’s The Vietnam War has shied away from tired tropes as it slowly and often exquisitely lays down the historical underpinnings of America’s most divisive war.

It’s in tonight’s exhaustive, emotionally grueling two-hour outing, “Resolve,” however, that the film — 10 years in the making — cuts close to the bone. The Vietnam War may be exhaustive, but it is never exhausting. There’s a moment in “Resolve” that is unspeakably sad, so much so that it kept me awake later that night on first viewing. To say more here would ruin the grace note — and for all its sadness, there is real grace here. That is what Burns does best in all his epic documentaries, but never more so than in The Vietnam War: He takes the wide sweep of history and imbues it with deeply personal stories of life and loss, featuring ordinary and yet remarkable people.

It’s 1966 when Burns and Novick pick up their story. More than 2,300 Americans have died at this point. Nearly 200,000 soldiers are stationed in-country, and more are on their way. The situation is difficult, US commander Gen. William Westmoreland has admitted, “and more difficult days lie ahead.”

“Resolve” focuses in the main on the widening rift between North and South, and the early signs of a burgeoning anti-war movement back on the US mainland. The night’s two hours head down the Ho Chi Minh Trail with terrified North Vietnamese volunteers, many of them teenage girls, as they smuggle arms and supplies under cover of darkness to their guerrilla comrades in the south, deep inside South Vietnam. The government in Saigon is determined to pacify the countryside, no matter the cost, and civilians on both sides of the demilitarized zone are being pulled deeper into the conflict.

Back in the world, as narrator Peter Coyote tells us in the opening moments of “Resolve,” hundreds of thousands of US soldiers are about to learn that “the war they were being asked to fight was not their fathers’ war.“

Burns is wise enough not to put too much stock in hindsight. He’s a storyteller, first and foremost. Hindsight is anathema to telling a good story. The Vietnam War is long — 18 hours in all — but it’s that very length that affords Burns the opportunity to tell an important story, moment-by-moment, day-by-day, giving it time to breathe.

This has the effect of slowly revealing how the soldiers on the ground and politicians, war protestors, and armchair experts back home lived it at the time, without the benefit of hindsight.

That strict adherence to time and place serves The Vietnam War well. This is a documentary about the past, yes, but the way Burns tells his story, it’s possible to believe events are happening in the here-and-now. In this way, the audience becomes more witness to history than passive observers.

That’s one reason Burns vowed not to use any song that was recorded before the time on the screen. “Resolve” takes in the years 1966 and ’67, and the songs — there are many more than on previous nights — are from those years or earlier. Bob Dylan, for example, penned “Masters of War” over the winter of 1962-’63, three full years before “Resolve” picks up its tale. Dylan’s lyrics were a protest against the Cold War nuclear arms build-up of the early 1960s, but those very same lyrics are applicable, too, to the Vietnam experience (“You fasten all the triggers / For others to fire / Then you set back and watch / When the death count gets higher”).

One of the most poignant, profound moments in “Resolve” is the honest, unblinking testimony of North Vietnamese Le Minh Khue, a “youth volunteer,” just 16 at the time.

Minh Khue was one of hundreds of thousands of volunteers, half of them women and teenage girls, who smuggled arms and supplies down the Ho Chi Minh Trail by cover of night, racked by fever and exhaustion. The Ho Chi Minh Trail was its American name; Minh Khue and her fellow volunteers knew it as Route 559, named for the men and women of the 559th North Vietnamese Army Corps. The Ho Chi Minh Trail pushed through an area the size of Massachusetts, “12,000 miles of tangled jungle roadways.” This was no nighttime walk in the park.

Minh Khue, a dog-eared copy of an Ernest Hemingway novel tucked into her backpack, was one of 230,000 teenagers who worked to keep the road open by day and the traffic moving at night, all the while ducking airstrikes that, in the end, dropped more than three million tons of explosives on the Laos section of the trail alone — a million tons more than all the  explosive that had been dropped on Germany and Japan combined during World War II.

The airstrikes had a new feature, too — the systematic spraying of a new, deadly chemical defoliant that would become known as Agent Orange, for the orange stripe on the 55-gallon drums in which it was stored.

An early black-and-white photo of Minh Khue shows a shy, round-faced girl who looks more 12 than 16. She idolized Hemingway. Hemingway, she thought, represented the best in America — which is telling, because one of Hemingway’s earliest novels, A Farewell to Arms, is a love story set during the days of the Spanish Civil War, in which leftist partisans fought a campaign against a fascist dictator-in-waiting. Minh Khue observed her 17th birthday on the Ho Chi Minh Trail, but it was no celebration.

“We all had to endure,” Minh Khue recalls in the film, through subtitles. “The jungle was humid and wet. Bombs fell day and night.”

Starvation and accidents, fevers and snakebites all took their toll. The bombing was relentless.

“We women had to find a way to survive,” Minh Khue continues. “We thought it was terrible. . . . We didn’t even have time to breathe.”

The Vietnam War shows how war pays no favors to either side: The North Vietnamese suffered terribly.

Again, that shows Burns’ integrity as a historical filmmaker: Most documentarians wouldn’t bother with the Vietnamese perspective.

In some ways, this is Burns’ most personal film. The idea that war exacts a toll from both sides, right or wrong, is what informs his work as an artist from the very first moments of The Civil War and has carried through his later war films.

The word “resolve” can be read many different ways. Burns and his writer, historian Geoffrey C. Ward, chose that word carefully when settling on an episode title.

“Resolve” means different things to different people. The resolve of an American president to keep pressing an unwinnable war; the resolve of US soldiers to keep faith and rely on their comrades in impossible conditions; the resolve of protestors back home to bring the whole thing to an end if their political leaders won’t listen to them; the resolve of a 16-year-old girl who worships Hemingway to brave night terrors to defend her country against the invader; and the resolve of a documentary filmmaker from Walpole, New Hampshire to summon everything he’s ever learned about filmmaking to shape the defining statement on one of the most turbulent, divisive moments in American history — it’s all here.

“I understood theoretically what it meant to be in a war,” 1st Lieutenant Matthew “Matt” Harrison, a squad leader with the 173rd Airborne, an elite unit dubbed “General Westmoreland’s Fire Brigade,” explains quietly toward the end of “Resolve.”

“But of course no one can really understand it until they’ve done it.”

There’s a catch-in-throat moment at the very end of “Resolve” that’s both poignant and profound, an emotional beat that informs the entire film, and a fitting coda over the end titles. If you see just one installment of The Vietnam War, this is the one.

TV Worth Watching will preview Episode 5: This Is What We Do (July 1967-December 1967) Thursday. The Vietnam War airs Sunday through Thursday on PBS (8 p.m ET), until Sept. 28 (check local listings).