How do you feel when you see a soldier? Does your chest swell with pride, or your gorge rise at the thought of what soldiers do for a living? These questions struck me recently, as I started work on reviving Britten's contentious "pacifist" opera Owen Wingrave, first performed – on television – in 1971. We'd decided to shoot the publicity photos in front of Charles Sargeant Jagger's Royal Artillery Memorial at Hyde Park Corner. Something about the uncompromising weight of his figures seems to resist all attempts to think about war in the here and now, not in some sepia-toned past. But just as we were setting up we were disturbed by the soft jingle of harness and the barking of military orders. We'd forgotten that the monument is on the route the Household Cavalry take twice daily to the changing of the guard. As they swept past, imperiously handsome on their great black steeds, the photographer scrambled to get them in the shot, and I remembered that this is a scene that Britten actually includes in his opera. The moment after his troubled young hero has announced that his pacifist ideals oblige him to throw over the chance of a place at Sandhurst and turn his back on his family's soldiering traditions, he retreats in shock to a bench in Hyde Park to gather his thoughts. The parade passes, just as they were doing now for me, and after a brief moment of dazzle at their beauty, he finds himself imagining just what their chosen profession involves – the mutilating of bodies and minds in the name of the state.

In this centenary year, the first poppy-red wave of first world war dramatisations has already broken over the TV schedules, and misty-eyed politicians' speeches have been made. Everyone has been carefully briefed to avoid the word "celebrations", but the temptation to unfurl a union jack has already proved irresistible for some. Clearly, we need to resist the cliches. This year, even the simplest of words and phrases should be scrutinised.

Wanting to be part of that scrutiny was why I accepted the invitation to revive Owen Wingrave. The piece has been dogged by its reputation as the black sheep of Britten's operas – too "difficult", too "outspoken", too unlovely in its musical austerity. The timing of this revival – the first night comes just before the centenary of the 1914 Sarajevo assassinations, and the last performance will be given the week after the centenary of Britain's official entry into the war – seems to provide the perfect opportunity to assess whether those supposed weaknesses are in fact the piece's greatest strengths.

Britten started work on the opera late in 1968, a year, like 2014, when war and its contested meanings were at the front of people's minds. The US invasion of Vietnam was reaching its tipping-point, and the skies over the composer's native Suffolk were noisy with US military aircraft. It was under the specific pressure of this crisis that the composer chose to respond to an invitation from the BBC to compose a new opera for television – and therefore for a potentially huge audience – by choosing to stage what he must have known would be a deeply controversial story. Henry James's 19th-century novella Owen Wingrave documents the last seven days in the life of a young man who defies his family's militaristic traditions and declares himself to be, in effect, a conscientious objector. It was, of course, a deeply personal choice of subject: as a young man, Britten had been an active member of the Peace Pledge Union, and his life-long commitment to the union's principles cost him dearly. Many people, fellow-musicians included, never forgave him for leaving Britain in 1939 and thus avoiding military service. At one point in the opera Owen's family express their reaction to his decision not to fight by repeatedly screaming the line "How dare you?" at him with great vehemence, delivering the line again and again, with the full orchestra backing them up in every vitriolic note. The scene seems almost impossibly overstated – until you remember that in April 1942, when Britten and his partner Pears decided to return to wartime Britain from the US, but with no plans to enlist for military service, they had had to argue their way out of summary near-arrest when they landed at Liverpool. Was that line, I sometimes wonder during rehearsals, spat into Britten's face that night on the docks by some irate interrogating official?

Owen has often been dismissed as too black and white. However, as the rehearsals gather momentum, it's becoming clear that the much-discussed "pacifism" of the opera is far from straightforwardly handled. As so often in Britten, it is in ambiguity that the music finds its true power to disturb. James's novella, like The Turn of the Screw, is a ghost story, and in the heart of the country house where most of the opera is set there is a haunted room that provides both the story's dark motivation and its bitter denouement. The ghosts that haunt it conform to a pairing that reappears throughout Britten's work; one is an abusive, murderous (male) adult; the other a murdered (male) adolescent. Referred to by Owen as "the bully" and "the boy", these two figures repeat the horror of Quint and Miles in The Turn of the Screw, of Claggart and Billy in Billy Budd, and of Peter Grimes and his hapless apprentices. But in contrast to those other pairings, the central act of violence in the opera is not an accident: the murderer was a father, and the victim his son, killed because he refused to fight in a playground squabble. Britten turns this central image on its head. Young Owen isn't haunted by the violence that was done to the boy so much as by the knowledge that, in putting on uniform, he himself would become the perpetrator of that violence. Throughout the opera, he battles to control his temper, continually lurching from sombre meditation to furious outburst. Owen's pacifism comes from the terror of knowing that he could so easily become one of war's perpetrators. Seen in this light, the opera's central character becomes one of Britten's most vivid and disturbing self-portraits.

One of the most dispiriting things about the commemoration of war is its endless repetitions – the recycling of the same images, the same feelings, the same laments. Britten's music embodies this brilliantly – every member of Owen's family is trapped in their inability to look at anything afresh. Crippled by the unholy trinity of grief, conformity and pride, they not only refuse to throw away these crutches but, outraged by the possibility of change, they brandish them as weapons. Then, right at the heart of the opera, Britten does something extraordinary. Matching his own courage to his hero's, he steps outside this vicious circle of repetition and grants his hero – and through him, us – a vision of what lies outside it: peace. The orchestration shifts, the stage clears, and for four glorious minutes composer and hero pour their hearts out in one of the most moving passages Britten ever wrote. Of course, being Britten, this vision of peace is troubled, provisional and fleeting. But it is also full of beauty. It is the sound of a man haunted by violence, but daring to look beyond it.

This extraordinary monologue has the music, the thoughts and the feelings that I ache to hear more of as the centenaries of 2014 roll on. In this year crowded with official commemorations, everybody needs a space in which to let their feelings about war escape. Owen Wingrave describes that space, and dares us to enter it.

• Owen Wingrave opens at the Aldeburgh festival on 13 June (aldeburgh.co.uk), and then transfers to the Edinburgh international festival on 15 August (eif.co.uk). Neil Bartlett and Kate Pullinger's Letter to an Unknown Soldier project is online at 1418now.org.uk/letter.