4 Stars

For whatever reasons, World War I doesn't figure nearly as much as World War II on the movie screen. Maybe that's because it was years ago and there have been so many other wars and quasi wars in between. Or maybe it's because in World War I much of the conflict was confined to the trenches for months on end, a scenario that can have limited visual and dramatic appeal.

Either way it doesn't really explain why the first global conflict hasn't been mined more for its story-telling potential.

Director Sam Mendes saw the potential. He collaborated with writer Krysty Wilson-Cairns to develop a script from a piece of family history. It was a story his paternal grandfather, Alfred, told him about a young soldier who was sent across enemy lines alone to warn a British battalion that they were heading into an ambush.

It's a slight story based loosely on fact. A race against time, a picaresque journey across the front, and a powerful re-creation of the conditions in the field for the millions of young men on both sides who sacrificed their lives.

1917 begins in spring on the edge of a grassy field, where lance corporals Schofield (George MacKay) and Blake (Dean-Charles Chapman) are napping in the sunshine. The countryside looks just like what tourists see today, broad, green fields that were once the battlefields of World War I extending as far as the eye can see. How could such a beautiful, peaceful setting have once been the backdrop to scenes of extraordinary carnage?

Their nap is interrupted. Blake suddenly gets orders to report to General Erinmore (an economical appearance from Colin Firth). He is told to take a mate along with him so his friend, Schofield, goes too.

Their secret assignment is to let Colonel MacKenzie (Benedict Cumberbatch, another celebrity cameo) know that although it looks like the Germans have retreated, they have laid a trap and any advance will end in a catastrophic defeat.

And there is a further incentive. If they reach the battalion in time, Blake will also be able to save the life of his older brother, a lieutenant. It's no coincidence that Blake sets the pace. The two men leave immediately, in broad daylight.

Schofield and Blake pick their way across no man's land and enter the trench system the Germans left behind. It is infested with rats and set up for unsuspecting enemy soldiers and they barely escape with their lives. They are safer travelling above ground, picking their way through the ransacked farms and burning villages to their destination.

From beginning to end, the camera of cinematographer Roger Deakins keeps the young soldiers firmly in its sights. One of the best in the business, Deakins has always been up for a challenge. He has shot 1917 in what appears to be a single continuous take. If you search for them you could probably pick the edits, but the overwhelming impression is of being swept up in the two-hour journey through the killing fields, carried forward without let up. Immersion is total.

It's the impression you get in other films shot in a single take, or something like it. Hitchcock's Rope is a famous one, though the cuts have been skilfully buried. Alexander Sokurov's Russian Ark is a more recent example, in which a trip through the Hermitage is actually managed in a single continuous shot.

Mendes' 1917 is the boy's own story. Neither of the fresh-faced leads, Chapman and MacKay, is as well known as the stars with bit parts, and that's as it should be. 1917 commemorates the generation of young lives that were lost to the war machine.

Some of the classic films about the 1914-18 war, like Jean Renoir's La Grande Illusion and Lewis Milestone's All Quiet on the Western Front in the 1930s and Peter Weir's Gallipoli in the 1980s carry a strong anti-war message. They Shall Not Grow Old, Peter Jackson's superb archival documentary on World War I released last year, sends a similar powerful message of the loss of young lives.

MacKay is simultaneously appearing in the lead role of Justin Kurzel's True History of the Kelly Gang, the latest film about the local folk hero who refused to take orders from anyone. Had MacKay's character Schofield gone home to England a far stroppier and anti-authoritarian lad, who could blame him?