There are few insights as shopworn as “war is hell,” but there’s a sequence early in Sam Mendes’s new WWI epic “1917” that brings that old chestnut into, particularly sharp focus. The film’s protagonists, Lance Corporal Schofield (George MacKay) and Lance Corporal Blake (Dean-Charles Chapman), men on a mission of utter urgency and desperation, have just stepped onto the “no man’s land” separating British and German troops in northern France. They’re crossing a battlefield, and until quite recently, an active one. It’s a wasteland; flies swarm the fallen horses, rats feed on abandoned bodies. During one particularly hairy moment, Scholfield finds his freshly-cut hand inside a dead man’s gaping wound.

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The parade of horrors in this sequence reminds us, surely not accidentally, of the flying limbs and other gross-outs in “Saving Private Ryan,” but they don’t exist purely for the sake of a jolt (well, not only for that reason). Classic war films were, in a strange way, feel-good pictures, reminding us that while war may be tough and lives may be in danger, those are small prices to pay for the sake of camaraderie, doing what’s right, and becoming a man.

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The modern war film, from “Apocalypse Now” and “Platoon” to “Ryan” and “Dunkirk,” operates on a more direct level, eschewing the careful choreography and flag-waving for sheer, moment-to-moment terror. In battle, you could die at any second. A random, nowhere bullet could end you before you even hear it coming. There are no rules, only survival, and survival is mostly about luck.

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Viewed through this prism, the much-ballyhooed gimmick of “1917”— that Mendes and co-writer Krysty Wilson-Cairns conceived it, and cinematographer Roger Deakins shot it, in long, continuous, mostly unbroken takes— seems less like a trick and more like a tool. The best of modern war cinema adopts a grunt’s perspective; the story isn’t happening in the bunkers, but in the trenches. Mendes observes much of Scofield and Blake’s mission quite literally over their shoulders, and alongside them for the journey, we lose the sense (subtly or even subconsciously engineered by compositions and cuts) of danger beginning or ending. The threat is relentless and could arrive at any time.

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This fact holds especially true of this particular mission, which both men realize is barely more than a suicide run. It’s April of, yes, 1917, during Operation Alberich; British troops are hoping to push the Germans further back from a retreat to the Hindenburg Line, unaware that it’s a trap for a massacre. Schofield and Blake are sent to deliver a direct order calling off that attack, and there is a personal stake here— Blake’s brother is one of the men who will fight, and die, if they don’t deliver that order. The clock is ticking and it’s a race against time with the odds duly stacked against them.

There are occasional appearances by famous faces – Mark Strong makes the most substantial impression, striding into his scenes as though he’s been on the front lines for years – but the picture is MacKay and Chapman’s to carry, and they shoulder it. When a film is constructed like this, with such careful planning, blocking, and camera choreography, it’s presumably a challenge to maintain a sense of spontaneity. But both actors convey their shock, fear, and desperation; MacKay is particularly good, especially as so much of his performance is pure reaction and non-verbal response. He has a scene, lost in thought on the back of a transport truck, where he sings an aria without saying a word.

Of course, he gets a hand from Deakins’ elegant photography; the light levels of that scene are so delicate, he feels perpetually on the edge of disappearing into darkness, without tumbling over. The Oscar-winning cinematographer gets a few showcases, put-a-frame-around-it moments like that (also of note: a stupendous sequence of night fire chaos that evokes both the imagery and hopelessness of “Full Metal Jacket”), but obviously and understandably, his main focus is less about composition than movement. As he shadows and precedes these men on their mission, snaking through the foxholes and dugouts, scaling embankments and zipping through firefights, we have to pause and ask: is this splashy style necessary?

Well, yes and no. No, in that it’s a story that could’ve been told in a more conventional cut-and-run fashion; no, in that it even an admirer must acknowledge considerable spatial and chronological elasticity; and no in the sense that, at a certain point, we can just throw up our hands and dismiss their achievement as showing off.

But there’s also a long, rich history of filmmakers showing off with their long takes and similar feats of technical wizardry, and so long as it’s serving the story in some capacity—as the long takes in “Goodfellas,” “Boogie Nights,” and “Children of Men” demonstrate—then why not? (As cinematic trickery goes, I’ll take this over the ponderous watch-winding of “Dunkirk,” personally)The proximity and intimacy of the technique render Schofield and Blake’s journey more visceral, and more frightening. And as a result, at its conclusion, the catharsis lands with the force of a hammer. [A-]