Last night, 'Dunkirk', the biggest, most eagerly awaited film of the summer, premiered at the Odeon Leicester Square. Prince Harry joined its stars, including Harry Styles, Mark Rylance, Tom Hardy, Kenneth Branagh and Cillian Murphy on the red carpet. So why is Dunkirk such a thrilling prospect?

The Director

'Dunkirk' is directed by Christopher Nolan ('The Dark Knight Trilogy', 'Inception', 'Interstellar') and it is his first historical film, documenting the evacuation of Allied troops from the beaches of France in May 1940.

It's a complete passion project for him. In the mid-Nineties, Nolan, with his producer and wife Emma Thomas, made a Channel crossing to Dunkirk with a friend in a small yacht, in the spirit of re-enacting the journey.

Due to bad weather, it was a difficult, dangerous 19-hour trip and it left a lasting impression on them.

"What stuck with me was just how extraordinary it was, the notion of civilians taking small boats into a war zone. They could see the smoke and fires for many miles, so their willingness to do that and what that says about communal spirit are extraordinary," Nolan has said.

The backstory

Nolan and his wife read first-hand accounts of the evacuation. They were puzzled as to why there had been no modern film of such an historic event that has so influenced our sense of ourselves and our spirit as a nation (not since the 1958 Ealing Studios classic, with John Mills and Richard Attenborough, directed by Leslie Norman, father of the late Barry).

In an interview for the tie-in book by the film's historical consultant Joshua Levine, Nolan says it's because it was not a victory, not even a battle, but a defeat - and getting Hollywood's resources behind a tale of a defeat is, to say the least "a little tricky".

Fortunately, Nolan, still only 46, has pretty much carte blanche (which makes his admission that he's tempted by doing James Bond intriguing). All his films have been immensely successful, not just the 'Dark Knight' Trilogy - 'Inception', for example, grossed $825m on a $160m budget. He didn't even pitch the idea to a studio initially, he just wrote a script - one of his shortest, at 76 pages, with relatively little dialogue. (The deal he struck with Warner Bros seems to be one of the best ever, £20m in salary plus 20% of the box office gross.)

Early on, Nolan decided on a triptych, "showing events from the land, sea and air: seeing the action from the perspectives of the men on the beach, the people coming to help on the boats, and pilots trying to protect them from above". Each element had different timing, the men being on the beach for a week, the boats crossing over a day, while the Spitfires had only an hour's flying time over the scene.

Nolan carefully braided them together, he says, "to allow you to understand the journey each character is on while always trying to suggest that there are many other unseen journeys" and no comprehensive overview.

In this way, a film as authentic as it can be about an historical event can nonetheless also be another of the mind-bending films about the subjectivity of human experience of time and the objective world that Nolan has given to us before.

The authenticity

He also wanted to make the film as immersive an experience as possible - and to this end decided we should not actually see any Germans, just as the men on the beaches being attacked rarely would have done.

In 2015, Nolan walked on Dunkirk beach with his production designer Nathan Crowley and realised they had to film there, at the same time of year, to make it as authentic as possible. Principal photography began last May. The breakwater that let ships come in to the shallow, tideswept beach was reconstructed. Real Spitfires flew (and really landed on the beach) and Nolan went up in one to understand what it felt like.

Among the small boats were many that made the journey in 1940 and have been maintained by the Association of Dunkirk Little Ships. A vintage French destroyer was towed up from a naval museum in Nantes.

Such is Nolan's determination to avoid CGI that, although 6,000 extras were used, they were supplemented by cardboard cut-outs of soldiers and vehicles, rather than special effects.

The stars

Having been shocked to realise how very young most of the soldiers on the beach were, Nolan cast young actors from the UK in the main roles, including 20-year-old Fionn Whitehead as Tommy, and a certain 23-year-old Harry Styles as Alex.

Emma Thomas says of Styles: "There's always a risk that people can't get past the persona but the truth is he's a great actor."

Those who managed to see the film before last night's premiere in London perhaps surprisingly agree.

There are veterans too. Tom Hardy is a Spitfire pilot, Sir Kenneth Branagh a naval commander, while Mark Rylance brings over a small boat and Cillian Murphy plays a "shivering soldier", who having escaped the beach does not want to return.

It has to be noted that historical accuracy, combined with this tight focus on Operation Dynamo on the beach, means this is an all-male cast (save for Miranda Nolan as a nurse).

The filming is radical, shot on large-format Imax cameras, despite adopting subjective points of view, so that we see what the pilots, soldiers and seamen see. The great cameraman Hoyte van Hoytema actually shouldered on of these things on a rocking boat.

Nolan was the first to use Imax cameras for a big feature in 'The Dark Knight' and he argues that "the immersive quality of the image is second to none. When you sit in the movie theatre the screen disappears and you get a very tactile sense of the imagery", not only in large panoramas and action sequences but in intimate scenes.

The soundtrack

Since there's limited dialogue, the score is key - and it is by Hans Zimmer, played as one long piece, incorporating sampled sound effects and responding to the story's timescales.

At Nolan's suggestion, it works around an adaptation of the Nimrod theme from Elgar's Enigma Variations, which he reckons to be "as beloved to the English as the story of Dunkirk itself".

The relevance

Is 'Dunkirk' just another war story, nostalgic for past greatness, its release ironically timed as we begin a different, less heroic retreat from Europe? Nolan doesn't see it as a war film but a survival story, the genre if anything being nearer horror, the story being "approached from the point of view of the pure mechanics of survival rather than from the politics of the event".

It's a universal story, Christopher Nolan contends. "Everybody can understand the greatness of it. It's the Israelites driven down to the sea by the Egyptians."

Then again, in the interview in Levine's book, he admits there's an all too contemporary resonance: "One of the horrible, unfortunate things with the migrant crisis in Europe is that we are dealing once more with the mechanics and physics of extraordinary numbers of people trying to leave one country on boats and get to another country.

"It's a horrible resonance but it's easy in our technologically advanced times to forget how much basic physics come into play. Reality is insurmountable.

"If you have a vast number of people in one place and they need to get somewhere else and can't fly and have to get on boats - to overcrowd the boats, with that human desire for survival… it's unthinkably horrible to see it on our front pages in this modern day and age. But it's there."

Dunkirk: The History Behind the Major Motion Picture by Joshua Levine is published by William Collins, price £8.99. Dunkirk will be in cinemas from Friday, July 21