James Crozier was 16 when he presented himself at his local army recruiting office in Belfast in September 1914. He was accompanied by his mother, Elizabeth, who tried in vain to prevent him enlisting. The recruiting officer, who also happened to be called Crozier, assured her he would look out for her son and "would see that no harm comes to him".

Throughout the winter of 1915-16, Private James Crozier fought on the Somme in the 36th Ulster Division, 9th Battalion Royal Irish Rifles. In early February 1916, he failed to report for sentry duty in the trenches near Serre on the Western Front. A week later, he was found wandering in a daze some distance behind the front line. An army doctor examined him and declared him fit in both mind and body and, on 14 February 1916, he was court-martialled for desertion. James Crozier defended himself, saying that he had not known what he was doing when he went absent and had been wracked with pains throughout his body. He was sentenced to death.

Frank Crozier – the officer who had reassured Crozier's mother – was asked to supply a recommendation as to whether or not the sentence should be commuted. He recommended that it should be carried out. On the eve of the execution, whether out of compassion or guilt, he insisted that the condemned soldier be plied with drink through the night. As dawn broke on 27 February 1916, 18-year-old Private James Crozier, a boy who had defied his mother to fight for his country, was carried, unconscious from alcohol, from a holding cell to the grounds of a commandeered villa nearby. As he was incapable of standing, he was tied upright to a post and blindfolded.

Private James Crozier. 07:05 / 27.2.1916. Le Domaine des Cordeliers, Mailly-Maillet, Picardie. From Shot at Dawn. Photograph: Chloe Dewe Mathews

His officer namesake later recorded the proceedings in his memoirs: "There are hooks on the post; we always do things thoroughly in the Rifles. He is hooked on like dead meat in a butcher's shop. His eyes are bandaged – not that it really matters, for he is already blind."

The firing squad, which was made up of soldiers from James Crozier's own regiment, shot wide and, after an army doctor confirmed that Crozier was still alive, an officer drew his revolver and fired a single bullet into the victim's head. "Life is now extinct," Crozier later concluded in his recollection of the execution. "We march back to breakfast while the men of a certain company pay the last tribute at the graveside of an unfortunate comrade. This is war."

Chloe Dewe Mathews at work on the Shot at Dawn project, Belgium, December 2013. Photograph: Jim Powell for the Guardian

Almost 100 years later, at dawn on a freezing December morning in 2013, the British photographer Chloe Dewe Mathews set up her camera in the grounds of a French chateau in Mailly-Maillet in Picardy. The resulting photograph is austere: a tangled bush stands in the middle of a green field which slopes upwards to bare trees and a grey wintry sky. This ordinary-looking landscape is imbued with a melancholic power because of what happened there on a cold February morning in 1916. It is the place where Private James Crozier was executed.

Dewe Mathews's series Shot at Dawn records many of the sites where around 1,000 British, French and Belgian soldiers were executed for cowardice or desertion (records of where German soldiers were shot were destroyed during the second world war). It was commissioned by the Ruskin School of Art at the University of Oxford as part of a commemorative art series, 14–18 NOW, and will be published as a book in July and exhibited at Tate Modern in November.

"Initially, I was wary of taking on a project about the first world war as I have no personal connection with it," says Dewe Mathews, "but, from a documentary photography perspective, I was drawn to the idea of arriving somewhere 100 years afterwards. It's almost the opposite of war photography. So, instead of the photographer bearing witness, it is the landscape that has witnessed the event and I who am having to go into that landscape in the hope of finding anything tangibly connected to the event. It was almost like having to find a new language or way of seeing."

She spent the past two years researching the project, meeting war historians and academics, slowly identifying the places where the executions took place. On her initial four-day trip to the first world war battlefields in November 2012, she visited Piet Chielens, the director of the In Flanders Fields museum in Ypres, Belgium. His area of expertise is the soldiers that were shot at dawn. After talking to him, Dewe Mathews found the project began to take shape.

Second Lieutenant Eric Skeffington Poole. 07:25/10.12.1916. Prison cell, Town Hall, Poperinge, West-Vlaanderen. Photograph: Chloe Dewe Mathews

"I had studied the first world war at school and knew about the terrible suffering and slaughter, but I had never heard about the executions and so I was really shocked," she says. "It seemed incredible to me that young men who had signed up to fight for their country and who were sent out to the trenches and exposed to this unimaginable horror, should be executed by their own men because something went wrong in their heads or they simply couldn't do it any more. From today's perspective and our understanding of soldiers returning from Iraq or Afghanistan with post-traumatic shock, it just seems brutally unjust."

On several lone trips, Dewe Mathews photographed the places where the executions were carried out – farmers' fields, woodlands, a primary school, a slag heap, a former abattoir… She also photographed the cellars and holding cells where the condemned men spent their last nights alive, one of which – the prison cell in the town hall of Poperinge, West Flanders – has scrawled messages still visible on the wall.

Poperinge town hall is rare in that it is one of the few execution sites that attracts large numbers of visitors on the first world war remembrance trail. Most of these emotionally loaded places have, until now, been lost to history, known only to locals old enough to remember the stories told about the executions. Many of the relatives of the executed men are unaware of where exactly they were killed.

"For a time, some of these places had an almost macabre fascination," says Dewe Mathews. "One man I met, who was born not long after an execution had happened in a yard on his family's farm in Loker, West Vlaanderen, told me how the event had lingered in the local imagination, and cast a kind of shadow over the land and the family for years afterwards." In that photograph, a solitary tree stands in a misty field where Privates Joseph Byers, Andrew Evans and George E. Collins were executed in February 1915.

Former Abattoir, Mazingarbe, Nord-Pas-de-Calais. Eleven British soldiers were executed here between 1915-18. Photograph: Chloe Dewe Mathews

On another trip, Dewe Mathews visited the local council maintenance office in Mazingarbe, Nord–Pas-de-Calais, which is situated in a former abbatoir where 11 British soldiers were executed for desertion between December 1915 and March 1918. She was directed there by Madame Dambrine, an elderly lady Dewe Mathews describes as "a local citizen historian who had researched all the killings and also told me where the soldiers were buried. At the council offices, though, no one had any idea of what had happened there."

Dewe Mathews describes her approach as "very slow and thoughtful." She decided early on to photograph each site at dawn, in keeping with the time that most of the men were executed, and, as close as possible to the actual date that they occurred. After a while, she realised that "I was placing my tripod around the same spot where the firing squad had stood and looking directly at the place where the victim was placed." It was, she says, "a solitary and sombre undertaking".

Often Dewe Mathews was told to look for a sloping field or elevated piece of ground as soldiers were often shot in places where bullets would be embedded in the earth if they missed the target. One such place was a grim-looking slag heap officially called Fosse No 4 in Ferfay, Nord–Pas-de-Calais. "Of all the sites, it was the most depressing and slightly sordid," she says, "while other places often had an air of melancholy or seemed slightly otherworldly at dawn. The whole project had a kind of slow, solitary rhythm of its own that was unlike any other project I have done."

Private Herbert Chase. 04:30 / 11.6.1915. Sint-Sixtusabdij, Proven, Westvleteren. Photograph: Chloe Dewe Mathews

The 23 photographs Dewe Mathews has produced are studies in stillness and absence. As such, they belong in a documentary-landscape genre that includes Joel Sternfeld's On This Site and Paul Seawright's Sectarian Murder, both of which chronicle the ordinary-looking places where extraordinary events have occurred. As Geoff Dyer notes in his essay for Dewe Mathews's book, her images may "bear a conceptual resemblance to Sternfeld's, but they are taken within the already charged zone of memory that is the Western Front. Each is a place where something happened, enormous and terrible in itself, but easily, perhaps deliberately, overlooked in the context of the larger cataclysms of the first world war".

James Crozier was one of 306 British or Commonwealth soldiers executed after being court-martialled for cowardice or desertion during that war. The often confusing circumstances that led to their courts martial and the ruthlessness of their punishments only fully came to light with the publication in 1989 of Julian Putkowski and Julian Sykes's history Shot at Dawn. The book listed all the deaths and, where possible, the events leading up to them. "The predicament that confronted many of these condemned men was quite pathetic," they wrote. "Often in poor physical health, these ill-educated, inarticulate individuals were frequently exhausted from the strains of constant horrific trench warfare which drained their resolve – and ultimately their lifeblood."

A sustained campaign by relatives of some of the men led to the granting of a collective pardon on 8 November 2006 and a Shot at Dawn memorial in their honour now stands in the National Memorial Arboretum in Staffordshire. Yet their executions – and the manner in which we should mark them – remain a contested issue among historians and the military. "It is genuinely difficult to understand the motives of the pardons campaign," wrote Cathryn Corns and John Hughes-Wilson in their book, Blindfold and Alone, arguing that there should only be pardons for those who were suffering from shell shock when they left their posts, while other soldiers who "were demonstrably guilty" of desertion "deserved the full rigour of the law by the standards of their time". Corns and Hughes-Wilson dismissed the campaigners as "shroud wavers" and the memorial as "a shrine to the unlucky, the shell-shocked and the rogues alike".

But the "rogues", too, had fought for their country in the appalling trenches of the Western Front. "The stereotypes of the deserter or coward are not always true and the term 'cunning deserter' is a complete misnomer," insists Putkowski, an affable and engaging character who has meticulously researched the individual histories of each executed soldier. "Each incident has a unique set of circumstances and relatively few men actually ran away from action." In an essay for Dewe Mathews's book, the historian Hew Strachan defines many of the victims as "men who were not yet legally adult, because they had enlisted under age; men who were suffering from what was then called shell shock; men who genuinely became lost in the confusion of battle; men whose courts martial were in the hands of officers who lacked legal training and so were not properly conducted."

A family photograph of Second Lieutenant Eric Skeffington Poole, centre, one of the few officers to be executed for desertion.

For the French, who sentenced more than 600 men to death by firing squad, the executions were a kind of public theatre, often taking place in wide open sloping spaces while battalions of up to 2000 men passed by. "The military logic was brutal and ruthless," says Putkowski. "A public execution was seen as an example to other soldiers: this man has been executed for desertion and it will happen to you if you do the same."

GS Chaplin, a military policeman who witnessed the execution at Arras on 31 October 1917, of Private JS Adamson, a 30-year-old soldier in the Cameron Highlanders, left this stark account in his journal. "Next morning the sun was shining and a touch of frost was in the air. I was sent up the road to stop any traffic and high up I had a bird's eye view. I saw the man brought out to the post, the firing squad march into positions, turn right and take up stand. I heard the report as they fired and saw the smoke from their rifles. They then turned and marched off. The officer with revolver in hand inspected the body then turned away. The dead man was then taken away on a blanket and buried in the small cemetery in the next field, it was over, I came down but it did not seem real."

Although between 80 and 90% of British soldiers found guilty of desertion or cowardice had their sentences commuted, for every soldier found guilty, as Putkowski puts it, "the human fallout was appalling". He cites the case of Eric Skeffington Poole, one of the few officers to be executed. A temporary second lieutenant in the West Yorks, Poole went missing on the day of a battle in which the regiment lost 225 men. Although the battalion medical officer noted that Poole's confused state of mind at the time was almost certainly caused by shell shock, and Brigadier-General Lambert recommended that the sentence be commuted, Poole was executed at Poperinge on 10 December 1916 on the orders of Commander-in-Chief, Field Marshal Sir Douglas Haig, who wrote in his diary: "Such a case is more serious in the case of an officer than a man, and it is also highly important that all ranks should realise the law is the same for an officer as a private." Poole was officially registered as having "died of wounds". Such was the family's distress that they asked for Poole's name to be omitted from the casualty lists that were given to the press.

For working-class soldiers, the human fallout could be even harsher. One of those involved in the pardons campaign was Sam Watts, 89, the nephew of Private William Watts who was shot for desertion on 5 May 1916. In November 2012, Private Watts was granted 10 days' leave from action but did not return to his battalion. He was arrested near his home in Liverpool on Christmas Eve, 1915, and taken to Southampton where he was detained in custody. He managed to escape and make his way back to Liverpool, where he was arrested again on 6 March. Watts cited family troubles and produced a doctor's note stating that he was unfit to return to combat because his lungs were congested owing to gas poisoning. Although a wayward individual with a record for various petty misdemeanours including drunkenness and failure to turn up for training, Watts had acquitted himself well in action in the trenches. During October 1914, more than half of his battalion had been killed or wounded in heavy fighting. Still, he was sentenced to death.

Sam Watts's anger about his uncle's treatment is undimmed. "He was a brave soldier who was executed by his own comrades despite the fact that he was suffering from shell shock and had been gassed. His pay was stopped the next day and his wife was left to bring up eight children in the slums of Liverpool. The stigma was such that she was hounded by the neighbours because he had been branded as a coward and a deserter by the army. That's how it was in those days. People did not understand what those men went through and, for decades, it was never talked about in the family."

When Sam Watts's father returned from the war, "shell-shocked and gassed", and learned of his brother's execution, he had a mental breakdown and was confined in Rainhill asylum, where he remained for more 20 years. "He went mute," says his son. "He didn't speak a word again. When his eldest son was killed serving in the desert in the second world war, we didn't have the heart to tell him. For me the pardons campaign was a campaign to end all those years of silence and shame."

After the collective pardon was granted, Sam Watts attended a service at Liverpool town hall where William Watts now appears on the memorial list. Alongside his name, though, are the words "Killed in Action". "What that tells me," says Sam Watts bitterly, "is that they are still lying about what happened because they are still ashamed of what they did."

One hundred years later, it is difficult to reconcile our contemporary understanding of war and its related traumas with our bafflement at the often ruthless way armies treated their own men in adherence to contemporary ideals of duty and discipline. Chloe Dewe Mathews's images, which she has titled simply with the names of those who were executed and the dates when the executions took place, are a stark reminder of one of the least-known aspects of the first world war. They comprise a memorial of sorts to the soldiers shot at dawn, many of whom, as the historian Richard Holmes wrote, "sometimes found, blindfold and alone, that courage which had earlier deserted them in circumstances we can hardly guess at".

The book Chloe Dewe Mathews: Shot at Dawn will be published on 14 July 2014 by Ivorypress. Chloe Dewe Mathews: Shot at Dawn was commissioned by the Ruskin School of Art at the University of Oxford as part of 14-18 NOW, WW1 Centenary Art Commissions. It will be shown as part of the Conflict, Time, Photography exhibition at Tate Modern, from 26 November.