It should go without saying that if you’ve never been to war, you can’t pretend to know what it’s like. Yet for those of us who never have been, the movies have created our image of war; they’re the closest thing to it that most of us are likely to get. I’ll never forget the first time I saw the “Ride of the Valkyries” helicopter attack sequence in “Apocalypse Now.” I was a college newspaper intern who’d talked my way into the film’s American premiere on Aug. 15, 1979, at the Ziegfeld Theatre in New York. The movie, in a word, was shocking. The sound of Jim Morrison singing “This is the end…” told you, from the outset, that the stakes were about something larger than one disastrous American military morass, and by the time the helicopter massacre arrived, the film had become a trip and a nightmare at the same time, one that let you feel the adrenaline rush of killing (which was rather obscene, but a rush nonetheless) and along with it the sting of death.

The great war films that emerged from the Vietnam era were mass-cultural touchstones, and two of them, “Apocalypse Now” and “Full Metal Jacket,” rank among my favorite movies of all time. Yet as astounding as they were (and still are), as cinema and as moral reckoning, I always felt, probably naively, that the lesson that lurked inside them — this is what war is, and if you ever suspected it wasn’t hell then perish the thought, because it really is hell — was one that the culture now embraced as a stone-tablet truth. Thanks, in part, to the primal power of movies, mere propaganda could never again disconnect us from that lesson.

Yet Sept. 11, 2001, changed the stakes. As the run-up to the Iraq War demonstrated, 9/11 was a crack in the universe that made America ripe all over again for lies about war. We now have a president who treats reality as unreal; propaganda has become the air we breathe. That’s the kind of atmosphere in which the essential quality of war can, over time, be forgotten. If a reckless president decides to use war as a measure of his power and ego, or as a tool to usher himself into a second term, then what’s going to stop him? The answer is nothing, unless the ultimate lesson of war is one that remains alive in the public.

In that context, it’s hard to overstate how much influence the movies have had on our collective perception of war. It was only 20 years ago that the overriding message of the Vietnam films — this is what war looks like: an existential bloodbath of whizzing metal and limb-severing cruelty — was first applied in cinema to World War II, in Steven Spielberg’s shattering masterpiece “Saving Private Ryan.” If you read, say, the works of Paul Fussell, you might have a sense of the Second World War as an inferno of chaos, but Spielberg, working at the height of his powers, was the film artist who seared that violence into our bones. In “Saving Private Ryan,” he gave us nothing less than a new mythology of World War II, one that was just as heroic as the old mythology (maybe more so) but that now took in the experiences that WWII veterans had always been notoriously reticent to talk about. He gave us a taste of what they really went through, and in doing so expanded our collective sense of history.

Now Peter Jackson has done something comparable for World War I. His remarkable documentary “They Shall Not Grow Old,” which takes archival footage of the First World War and applies a panoply of techniques (colorization, speed changes, added sound) to those filmed records to make them look and feel contemporary, without in any way violating their essence, is a staggering film to watch. It’s the most vital piece of war cinema since “Saving Private Ryan,” because it reminds us of the thing that we so badly need to hear right now: that war, no matter how much valor we associate with it, is also something that stands apart from valor. It’s courage under fire, but it’s also torn-up bodies in a pit of mud. It’s a hell we don’t consider because to do so verges on the intolerable.

Jackson worked on this movie for four years — it was co-commissioned, in 2014, by 14-18 NOW and Britain’s Imperial War Museum, in association with the BBC, who first approached Jackson with the stipulation that he make use of the museum’s archival footage in an original way. The movie opens with grainy un-retouched footage accompanied by audio excerpts of WWI veterans recalling their experience of the war, and it’s eerie to hear their stiff-upper-lip attitude of gung-go good cheer; we truly seem to be listening in on a different world. They went into the war believing in it. Yet that belief now stands as both inspiration and warning. World War I was the first mass war ever fought with the new technology of destruction, and Jackson’s film chronicles how the veil of innocence of the men who went off to fight it was ripped away.

The footage stays grainy, herky-jerky, and black-and-white for the first half hour, during which we see images of British soldiers preparing to go off to the front lines, submitting to the rituals of basic training, which at that time had a vastness to them — 500 men marching down a road in formation — that, we can see now, still had one boot in the 19th century. The movie’s images convert to color, with a lifelike fluidity and flow, and the soldiers’ voices dubbed in with uncanny verisimilitude (lip readers were used), only after we’ve arrived at the theater of war. At that point, we look at these soldiers’ faces, and they no longer resemble distant scratchy replicas of people in old newsreels. They look like us; they could be us. The message of “They Shall Not Grow Old” is that they are us.

Their experiences come alive, through the transfigured verité images accompanied by a soundtrack of World War I survivors, and the result is like a combat diary that grows darker and darker. It’s all presented matter-of-factly, so that moments of camaraderie and even excitement in the fabled trenches rustle up against tales of sleep deprivation and gangrene, corpses eaten by rats, and sniper’s bullets that strike out of nowhere.

In the history of war cinema, Stanley Kubrick’s “Paths of Glory” has always been celebrated for its groundbreaking depiction of the horrors of trench warfare, but I’m sorry, as good a movie as that is, its authenticity is reined in by a certain 1957 studio-system aesthetic classicism. It’s a powerful drama that doesn’t necessarily scrape raw nerves. (“Full Metal Jacket,” made 30 years later, does.) Watching “They Shall Not Grow Old,” we’re in the trenches, along with those men, as much as we ever could be. The whole point of the war, the reasons that it was fought, remain off camera, and maybe that’s part of the point — that to the soldiers, the war stopped having a point. (In the film’s closing minutes, they say as much.) The violence, the squalor, the survival was all.

Jackson has done a brilliant job of bringing all this to life. When we see footage of land-mine explosions from a century ago, now shown at proper speed, with accompanying sound, they look as epically scary as nuclear blasts. Apocalypse then, apocalypse now. Yet as much as “They Shall Not Grow Old” honors the courage of the soldiers who walked, open eyed, into this snake pit of machine death, the message of the movie, to the extent it has one, is that this happened before, and it could happen again. Our key world leaders are now authoritarians out of central casting, and that includes the leader of the free world, who’s explicit about the fact that he no longer stands for freedom. He stands for saber-rattling, delusions of grandeur, and the violent hatred of “outsiders” — the very cocktail of war. That his darker impulses will stay submerged is by no means forgone. But the best way to rein them in is for the rest of us to never forget what war is. “They Shall Not Grow Old” is an annihilating testament to what it always has been.