Keep Calm and Carry On may be a World War Two slogan but it was formed from the strength of purpose that made our nation prevail in World War One. It is this quality of endurance above anything else that Steven Spielberg captures in his new film, War Horse.

"The most important thing in any trying times is never to give up hope," says the director. "Once you give up hope, you give up your soul."

Based on the book by St Albans-born author Michael Morpurgo, War Horse essentially is the story of Albert, a boy who leaves behind the quiet life of a country farmyard to find a friend in the sodden trenches of the Somme; only in this case, it is not a human comrade he seeks, but a remarkable animal with whom he has formed an unshakable bond. The original book is told from the point of view of Joey, the horse, and the film-makers have remained faithful to this by showing how this horse changes the lives of several people during the passage of its life.

“It is a love story and that makes it universal,“ explains Steven Spielberg. “We did a lot of research. What struck me was the vast number of casualties of people and animals in the Great War. It was the death knell for the horse as an instrument of warfare and an experimental war too, with the advent of tanks, chemical warfare and the aeroplane.“

Filmed in Devon with footage shot on the Luton Hoo Estate, War Horse is a truly British film – the most completely British movie Spielberg says he’s ever made. It stars Emily Watson as Albert’s mother, Benedict Cumberbatch as Major Stewart and newcomer Jeremy Irvine as Albert.

“We saw hundreds of potential actors for the role of Albert, but he had that ineffable quality to stand out above the rest,“ adds Steven. “Nobody else had the heart, spirit and communication skills that he had, even in silence.“

Fieldways Farm in Rickmansworth is where the actors trained in horsemanship. The farm, owned by the Dent family, has a long history of supplying mounts for movies. Their horses, period coaches and fittings have appeared in The Adventures of Black Beauty, Elizabeth The Golden Age, Gosford Park, Van Helsing, Cranford and Robin Hood, to name a few. The farm’s 80 horses are also highly trained to respond to their riders or to play dead on demand.

“The sensitivity and nobility of those animals is something I had never expected,“ says actor Tom Hiddleston who plays Captain Nichols, the officer who buys Joey and takes him to war. “Michael Morpurgo must have always known horses to write such a story, they have so much to teach us. We had five weeks training on the farm in Rickmansworth and our sessions would start at 8am, but Jeremy had been there since 6am mucking out. He was definitively the John Wayne among us.

“In the charge scene, Spielberg said: ’I don’t want you to do shock or fear, give me your noble face and then when I say the word ‘guns’ I want you to go from being 29 to nine. I want you to strip away the man and show me the boy. Can I leave that with you?’ That was the most heartbreaking piece of direction I have ever been given.“

This episode in a film that features so many poignant moments, stays with you long after you leave the cinema. The enormity of what this film deals with is actually too vast to have immediate impact – yes, you cry when people and animals are seen to suffer but the message grows and strengthens with time. A comment on the futility of conflict, it also tells of the courage, spirit and selflessness of people and animals facing impossible odds.

Michael Morpurgo speaking after Sunday night’s UK premiere attended by the Duke and Duchess of Cambridge, is clearly happy with Steven Spielberg’s majestic adaptation.

“He’s kept close to the spirit of the book,“ says Michael. “I hope the experience leaves you desperately sad, wrecked really because of the horrors of World War One. The charge scene, where we’re looking down on the battlefield and seeing the horses and men lying there is all that needs to be said. The pity. The waste.

“Any story you write about war is going to be political. I’m a war baby. I grew up in London after the Second World War and one of my earliest memories is of the city in ruins.

“You come out of that cinema, read that book or see it in the theatre and anything that gets us asking the question why – was it necessary? Anything that gets us thinking, particularly for young people, is really important.“

War Horse (12A) opens on general release on Friday, January 13.