They Shall Not Grow Old review: "a memorial that resonates like no other documentary"

It's narrated in a collage of commentary by unnamed soldiers (each one formally credited in the end credits with regiment and rank), their voices crackling with age and their spirit undimmed.

The pre-war build-up is one of patriotic duty and the sense of a national outing for young men: one volunteer compares the training and anticipation to a “holiday camp”. A string of then-underage lads cheerfully describe the dodgy methods by which they circumvented the 18-and-over requirement (one, aged 17, is sent away by a recruiting sergeant and advised to “have a birthday and come back”).

Jackson is best known for visualising verdant fantasy (Lord of the Rings, The Hobbit), but here he rises to the occasion of cold, hard reality. He makes sound documentarian decisions such as presenting the opening sections in Blighty in a square frame in misty black and white, with the constant whirr of a projector, as if showing home movies. Then he dramatically drops us across the Channel and into the quagmires of France and Belgium, with the reality of mechanised trench warfare revealed in widescreen and living colour.

You quickly get over the shock of seeing once-grainy monochrome newsreel tinted and rendered corporeal with the added sound of rumbling ordnance, jolly chit-chat among comrades in arms and even, during one serene interlude, the subtle clink of beer bottles during a singalong. To use the technical term for added sound effects, this is Foley of the highest order. Slates dislodged from the roof of a farm building by the thunderous retort of a howitzer are the perfect instance of a little sound effect going a long way to bringing a sequence alive.

The description from the soldiers is humorously colloquial at first, with talk of the “mad minute” of ten rounds being fired off in training and watered-down mess beer nicknamed “one pint and nine piddles”. But the gallows humour switches off permanently when battle comes down, and testimony is suddenly about noses “filled with fumes and death”, and a dead comrade’s perfectly bisected body providing “an excellent anatomy lesson.”

The big push towards the end of a war that claimed 9.5 million military and civilian dead among the Allied Powers and 8 million among the Central Powers proves harder to illustrate, with less available moving footage, but Jackson fills in the gap by augmenting scant newsreel and gut-wrenching commentary with elegantly idealised sketch illustrations from the time.

The sourest note to all this needless, empire-building carnage is sounded in the postscript, after what’s described as an Armistice without celebration among the ranks.

When returning to the land of hope and glory they had left four years previously, these damaged “Britishers” were met with a wall of indifference from a country now preoccupied with mass unemployment. The civilian population was ill-equipped to talk about what the servicemen had witnessed and endured, and, in general, the effects off war were underestimated.

Jackson and his team of technical, restorative historians have created a memorial that resonates like perhaps no other documentary on the subject – with specific honours offered in the final captions to Jackson’s serving English grandfather.

The volunteers and conscripts who, at the start of the war believed that it would be all over by Christmas 1914 ended it mentally and physically scarred, hoping that it was the war “to end all wars”.

With this account of events, Jackson ensures that their voices will inspire conversations about war and peace for generations.

They Shall Not Grow Old will be on BBC2 at 9:30pm on 11 November and back in cinemas later in the month.