Wars are inconceivable things. World wars with their millions of dead sit beyond even the wildest conception and, as such, great battles and specific events within them become the symbolic units of a nation’s understanding of itself at war. Each nation remembers wars differently, and for all the battles it fought in – the war in North Africa, the East, and even Italy – Great Britain tends to remember itself in the Second World War through four key events. The British have absorbed as emblematic of their national character the London Blitz, the Battle of Britain (fought in the skies), the D-Day Landings and Dunkirk. All have the same core mythos: on each occasion Britain was outnumbered and outgunned, but survived though pluck, luck and, as we understand it, a natural stoicism that became a latter-day embodiment of the British stiff upper lip.

Dunkirk should have been a tragedy. In 1940, the British Expeditionary Force had been routed from mainland Europe by the Nazis. They had fallen back to Northern France. 400,000 soldiers trapped on the beach of a seaside town bordered on one end by the cruel sea, the other by the overwhelming force of an approaching Nazi army. The sea was too shallow for major vessels to approach. There was nowhere to go and no rescue. Never before had a major armed force been so ill-equipped to defend itself. The Germans knew it and, knowing the British had no options, moved slowly to the kill. They took their time to build up an overwhelming slaughter, trapping the British in a small pocket and bombing them from the sky. Improbable events make wonderful stories. And this event, that British people will know but the rest of the world may not, turned on the most unlikely of circumstances. 700 civilian vessels – pleasure cruisers, sailing dinghies and fishing boats – led by weekend captains and hobbyists, crossed over from ports in England and made to the beaches to load up with the soldiers before the decisive blow. By the end of eight days, 338,000 soldiers were rescued. It was as near to a mass miracle as the degradations of war will allow.

It has rarely appeared in film. Leslie Norman directed a version of it in 1958 (also called Dunkirk) and most notably in recent times it serves as the backdrop to Joe Wright’s Atonement. It is a mass event, deeply cinematic in scope, and its portrayal would require a significant budget. Such budgets are rare enough for war films in this solipsistic age; they are near impossible for films that don’t feature Americans. Dunkirk happened in 1940 and America had yet to join the war effort; Japan had yet to bomb Pearl Harbor and the Americans were still unclear whether to support an ailing Great Britain or enter into an accommodation with Hitler. It was a time of American pragmatism rather than heroism (in fact America joined the war in Europe only after the UK handed over its gold stocks and all overseas ports sitting near the American East Coast to buy their involvement).

Pulling a $150 million budget together for a war film requires a director the studios can trust. One without a role for a Brad Pitt, or a Channing Tatum, even more so. No one expects the kind of return that Transformers delivers. But a steady return, bolstered by the cachet of awards, would suffice. Step forward Christopher Nolan; Batman brought the fame and Interstellar convinced the studios he could do no wrong. He had bought some latitude.

Nolan’s Dunkirk has been described as his masterpiece. A canvas for a great director’s singular vision. Dunkirk isn’t that. It is a good film threatening greatness, then retreating from that singularity of vision because in the end someone paid $150 million for the film to be made and such a compact with the devil of finance means that singularity of vision must kowtow to the necessities of standard narrative storytelling. This is a film of two halves.

The first thirty minutes of the film are undoubtedly exceptional. We meet Fionn Whitehead’s Tommy, an ‘everyman’ soldier cantering through Dunkirk’s war-torn streets pursued by the Germans (throughout the film they are never seen). Life and death is a throw of the dice. Others are gunned down around him but he is lucky enough to make it through the French lines to the beach. And what a beach it is. Nolan’s Dunkirk beach is an expressionist nightmare. Soldiers form orderly lines to the sea, waiting on ships that aren’t coming. At this time they are still hoping for ships and know nothing of the little vessels setting sail to their rescue (how could you ever imagine that your rescue will in the end be handed to you by 700 pleasure boats?) The passivity is jarring – they are forming literal orderly lines to nowhere, from the top of the beach to shoulder height in the sea.

Joe Wright’s beach in Atonement, drawn from actual accounts and pictures of the event, is a whirling Mardi Gras of desolation told in a nine-minute steady cam shot. It is Dante’s Inferno with sand, drink, singing choirs and horses that need to be shot. It is visceral, a 20th-century Hieronymus Bosch – a distant cousin to Aleksey German’s Hard to be a God or Coppola’s Apocalypse Now. War is a blistering, fetid hell presented with an almost fetishistic eye for depravity. The beach is muck strewn, the air grim with smoke

But Nolan is telling us something different. His beach is clean, sanitised (there’s barely blood the whole film). His soldiers are so passive it is hard to believe they are at war. And perhaps that’s the point, they aren’t at war. Nolan has taken the point of inaction, the shock of nothingness before abject panic, and stretched it thin and flat as a Normandy beach. The soldiers in Dunkirk are frozen in a moment, suspended in the ferment of their own emptiness, and with artistic latitude Nolan pulls that moment taut. It is a conceit, bearing no proper resemblance to actual events. Its power comes from its irreality; this isn’t war, this is a dreamscape experienced through sleep paralysis. Its natural comparators are Malick’s long grasses in The Thin Red Line, the Playboy bunnies scene in Apocalypse Now, or Yossarian’s crashed plane dream sequence in Mike Nichols’ Catch-22. It is a striking choice and deeply successful on its own terms.

Hans Zimmer’s soundscape (we’ll call it that rather than soundtrack) for the first half of this film is one of the greatest achievements in contemporary cinema and he will be feted and win awards for it. If Nolan has stretched psychological time on the beach as taut as a pulled string, Zimmer is playing that same string with an escalating and pernicious intent. Metronomically scratched violins build tension it is hard for the audience to believe they will ever be released from. Add to that the unforgettable sound of bombers harrying down on the beach and this film finds its moment of high art. We are, we feel, in the hands of artists as they deliver a hell less fetid than other portrayals but all the worse for the dreamlike paralysis it engenders – Run? Run where? Hide? Hide where? The vision is all the more potent for the lack of spoken lines; in its early phases the film is near silent and all the better for it.

There is no doubt Nolan is an artist. But he isn’t a great one. There is no doubt he is relatively singular in his vision but he is no Herzog, Kubrick or Kurosawa (to randomly name auteurs whose visions are, for good or bad, stretched from one end of a film to the other). It is possible Nolan may be a director desperate to impart his own style curtailed by the cruel Hollywood system. But I think it is more likely he is a director trying to butter his bread on both sides. Truly visionary directors can’t seem to help themselves even when their style risks sinking the ship. Nolan knows that he has to toe a line, and the sequence on the beach, the beating, painful heart of this film, uncomfortable, jarring and brilliant, gives way far too quickly to a series of improbable Hollywood narratives and standard tropes. Tommy, our everyman, who stands as our bewildered portal into the scenes of the beach (I see a parallel, for example, with Kyle MacLachlan’s character watching from the closet in Blue Velvet), slips for the rest of the film into a kind of heroic picaresque. He starts well, acting as a stretcher bearer in hope of joining a ship, but by the time he has survived the sinking of two ships (or is it three, it becomes hard to tell!), and endured a needless (and rather poorly acted) sequence in the darkened hull of a beached fishing boat, we have moved from high art to improbable sentimentality.

All the other elements of the film form the same Hollywoodised concoction. None of it is bad and it is genuinely exciting and moving but has the feel of a cocktail that would be much more pleasing if it weren’t so sweetened. Tom Hardy, acting with his eyes and doing it well, is a lone Spitfire pilot over the ocean. He is short of fuel, but by the end of the film he has knocked four German planes out the sky (all at just the right time!). Mark Rylance is a weekend boat captain, making his way to the beach, stopping to pick up Cillian Murphy’s shell-shocked soldier on the way (Rylance has a deep believability to him, Murphy less so). Kenneth Branagh dials in a performance as a stoic admiral in charge of the evacuation. People are endlessly rescued just in time. We slip away from the cadences of Herzog, Kubrick or Kurosawa (again, to randomly name those who have produced cinematic high art) and find ourselves beholden to the efficient, trusty cinematic cadences of a Spielberg or Zemeckis (or at least Nolan’s approximation of them). By the end of the film the celluloid bathwater is drained and the baby slips off down the plughole. A film that started with no talking, ends with a common solider reading a newspaper printed version of Winston Churchill’s ‘We shall fight them on the beaches…’ speech. The gnawing strings give way to generic adagio that could fit in any one of a thousand other films. All these things are losses, but nothing is more tragically let go of than that vision of Dunkirk as a paralysed hell.

In Saving Private Ryan, Spielberg starts with a 45-minute sequence that bucked the trend of all cinema to that date. Yes, it was visceral; yes, it was horrible; but more than anything, Spielberg told us that capital-H heroism was meaningless. In the chaos of a battle it doesn’t matter who you are, how good a shot you are, how tough you are, how fast you are, or strong, bullets will hit you and kill you all the same. Survival is a haphazard dice throw. It was blisteringly disenfranchising to an audience brought up on films where the central message is somehow always that plucky bravery will see the main protagonist through; his bravery and will-to-live will form a shield around him and the bullets will miss. However, by his own failing, as quickly as Spielberg established this fact of hopelessness, he jettisoned it in favour of a rather standard remaining story about a dauntless collection of soldiers enduring through bravery.

Dunkirk does something similar. It tells us first that war is a paralysing madness, and while it is doing so, we are captivated. But it stops, it gives us thrills and spills – the actual opposite of the paralysis! If Nolan is ever to be capable of a masterpiece he will have to find a formula to take Hollywood money from investors but produce something with far fewer Hollywood tropes. What if the film were silent the whole way through? What if it followed no real protagonist, let alone one who can survive two sinking ships? What if that unsettling soundtrack endured throughout? What if we were left paralysed on the beach, lost in a hateful dream? Well, probably this film would discomfort us and people would choose not to watch it in great numbers. But it would have achieved something that transcends, and Nolan doesn’t really have a will to do this. It’s possible he will always compromise, and Dunkirk is a good film that chooses an accommodation with the traditional values of Hollywood over genuine art by the end. If Nolan has a singular vision he doesn’t fully trust it and by the end of the film we are left confused. If what we are seeing is a frame for standard thrills and spills why have we deviated from the fact? Where were the 400,000 men on the beach? Where were the RAF (who flew 3,500 sorties over Dunkirk fighting to save the men but were portrayed here by three planes and mostly Tom Hardy’s super-duper Spitfire)? Where were the 700 boats (for most of the film anyone not knowing the story would think Rylance was the only captain who had set sail)? Aiming at first for art, and then compromising for a more traditional conception of the individualised heroic action, means neither strand is truly fulfilling.

See Dunkirk for its exceptional first half, see it for Tom Hardy in the sky and Mark Rylance on his boat. But don’t see it because you think you will experience Dunkirk, and don’t see it because you think you will see a masterpiece.