What passing-bells for these who die as cattle? Not only the monstrous anger of the guns nor the stuttering rifles’ rapid rattle, but now an epic four-minute, eye-wateringly expensive commercial for a supermarket chain.

There is no disputing that the new Sainsbury’s ad is, on its own terms, a masterpiece. The cinematography is breathtaking. Without saying a word, the young cast conveys a startling array of emotional depth within a few short minutes. The simple narrative, built around the near-mythical Christmas truce between the trenches of 1914, has just the right blend of poignancy and sentimentality to bring a tear to the most cynical eye.

So why does the advert leave me feeling so unsettled, so uncomfortable, even a touch nauseous? The first answer has to be that for all the respectful tone, the centennial occasion and the endorsement of the Royal British Legion, the ultimate objective here is to persuade us to buy our tinsel, our crackers and our sprouts from one particular supermarket. Perhaps the greatest truism, even cliche about the first world war is that nobody ever really understood why it happened, what it was all about. Those in the trenches never understood what they were giving their lives for, beyond a vague commitment to king and country, and a hundred years later most of us still don’t really understand. The sheer futility of the slaughter is what made the truce possible and also what makes the recounting of the tale so heartwarming and heartbreaking to this day. That vacuum of sense provides all the more reason for caution when co-opting the events for a purpose as crass as flogging groceries.

Of course many film-makers, writers and other artists have made good money from representing the horrors of war. Some do so with respect and artistry, some exploit shamelessly. But there is a key difference, I would suggest, between selling representations of war as a product, and using representations of war as a means to another end. Somewhere close to 40 million young men were killed, lost or mutilated in the first world war. Sainsbury’s has all but dressed them in a sandwich board. Donating profits to the Royal British Legion from the sale of the special chocolate bar that appears in the advert doesn’t change that.

In helping to launch the ad today, the legion’s head of fundraising praised the film’s historical accuracy and attention to detail. How true can this be? While there were certainly varying conditions on the frontline from place to place and year to year, reading contemporary accounts from either side of the trenches, in the poetry of Owen and Sassoon or the prose of Remarque, the details that stick in the mind are horrific. Nowhere in the new advert do we see the blood and entrails, the vomit and faeces, the rats feasting on body parts. The response might be “well they can hardly put that in a Christmas advert can they?” and that would be entirely true. Which is why the scene is entirely inappropriate for a Christmas advert in the first place.

The trench warfare of 1914-18 sits near the top of the list of horrors that humanity has inflicted upon ourselves and each other. Although it has recently slipped out of the range of living memory, it remains an iconic scar. Like the Nazi Holocaust or the dropping of atomic bombs on Japan, it lives on as a vivid phantom in our culture, a constant reminder of our capacity to inflict incomprehensible degrees of violence and suffering upon innocent individuals. It surely behoves us as a society to retain those deaths with respect and a degree of reverence. Would we welcome an advert next Christmas showing a touching little scene between a Jewish child and a disabled child in Auschwitz, swapping gifts for Christmas and Hanukah on their way to the gas chambers? I would hope not, yet I fail to see any great moral difference.

Exploiting the first world war for commercial gain is tasteless. This, however, is not what disturbs me most. The really upsetting details are the stunning shot of the robin on the wire, the actors’ trembles as they cautiously emerge from the trenches, half expecting a sniper’s bullet, the flicker of understanding in the eyes as the young soldiers reach into their pockets at the end. The film-makers here have done something to the first world war which is perhaps the most dangerous and disrespectful act of all: they have made it beautiful.