The war would not leave Tempest. Although he was demobilized in February, 1919, the Old Comrades Association commissioned him to write the history of the battalion between 1915 and 1918. Tempest borrowed the unit’s diary—a day-by-day chronicle of its actions, often scribbled in pencil by a junior officer—and placed advertisements in Bradford newspapers, asking to speak to other surviving members of the Sixth. (One appeal, in the Yorkshire Evening Post, acknowledged that many soldiers had disobeyed the order not to keep diaries while fighting in the war. “These are now required by Capt. Tempest to assist him in his work,” the text noted, adding, “There is now no danger of prosecution for any breach of regulations by producing them.”) Given the granular detail in his account, it seems clear that Tempest had himself taken notes or kept a diary during the war. His approach to the book project was to marry the factual fidelity required of an official battalion history with a novelistic evocation of life on the front. To enrich his narrative, he made a pilgrimage to the battlefields where the Sixth had so recently fought. His private album shows photographs of the tree stumps that remained of the woods near Aveluy.

While Tempest worked on his book, another officer, Herbert Read, was finishing his own. Read had fought near where Tempest was deployed, carrying a copy of Thoreau’s “Walden” in his knapsack; he eventually became one of Britain’s most influential art critics. In 1919, Read attempted to find a publisher for “In Retreat,” his short, arresting account of the Spring Offensive of 1918. Nobody would print it. The problem, Read came to believe, was that discourse about the war was mired in “sentimental illusion: it was a subject for pathos, for platitude, even for rationalization. It was not yet time for the simple facts.” In the first years of peace, there was, Read wrote, a “dark screen of horror and violation” that separated men who had served from the noncombatant public. It was only in the late twenties that the now famous prose works about the war—Edmund Blunden’s “Undertones of War”; Robert Graves’s “Good-Bye to All That”; Siegfried Sassoon’s Sherston Trilogy; and, in translation, Erich Maria Remarque’s “All Quiet on the Western Front”—began to be published, reaching a wide readership. (Read’s book was published in 1925, and achieved modest sales.)

Tempest’s book, delivered in 1921, is in many ways a document out of time. Its deadening title, “History of the Sixth Battalion West Yorkshire Regiment,” belies its emotional power. Within its pages, one senses a writer who could have added his name to the canon, had he wished. But Tempest may have been temperamentally unsuited to such fame. He appears almost entirely without authorial ego. In Julienne’s later years, she often told the story of how her husband was asked to use a photograph of himself on the book’s cover. He declined.

While Tempest was writing his “History,” Louis and Antoinette Thuillier stopped taking photographs. With the war over, the exercise now seemed pointless. Louis descended into depression. He was apparently tormented by the cache of images he had collected, in part because he did not know which of his subjects were still alive. In 1931, he fatally shot himself. The glass plates, now obliquely connected to the shame of Louis’s suicide, were stashed in a farmhouse attic and rarely spoken of again.

Some printed Thuillier photographs remained, but because they had been made rapidly and cheaply, the images were often hazy, and most faded quickly. Some prints did not last even a year. Even when the portraits were reproduced onto regular paper, few survived the century. Tempest’s book contains at least two Thuillier images: group shots of the Sixth in 1916, before the Battle of the Somme.

In 1988, a Paris antiques dealer named Laurent Mirouze visited Vignacourt. He was following a tip from a friend, who had seen striking images of First World War soldiers hanging in a municipal hall there. The town was celebrating the renaming of a pair of local roads in honor of Australian troops, on the seventieth anniversary of the Armistice. Mirouze asked Vignacourt residents who had printed the images, and he was directed to a photographer, Robert Crognier, who had made reproductions, apparently from the original plates. Crognier was the nephew of Louis and Antoinette Thuillier. He told Mirouze that the twenty or so pictures in the Vignacourt hall were a fraction of a huge collection that still belonged to the family and that was stored somewhere in the village. But Crognier would not divulge where the plates were kept.

Mirouze soon recognized that the Thuillier collection likely had historical significance: no other collection of First World War photographs was as fine or as comprehensive. He contacted Australian diplomats and British archivists, but nobody seemed interested in the collection. By 1991, Mirouze had abandoned his search. In 1997, Crognier died.

Then, in 2010, Ross Coulthart, an Australian investigative journalist, called Mirouze about the photographs. “I’ve been waiting twenty years for this,” Mirouze told him. Hundreds of Australian soldiers had spent many months in Vignacourt, and they had sat for nearly as many portraits as the Brits. Mirouze said that he would help Coulthart and an Australian historian, Peter Burness, track down the glass plates. After contacting various Thuillier relatives, the investigative team discovered that there was a rift in the family, and that some members didn’t want to relinquish the negatives, feeling that the French government seized First World War memorabilia without adequate compensation.

Eventually, the trio met with Crognier’s widow, Henriette. When they told her that their aim was to honor the Australian dead by displaying the Thuillier pictures at the national war memorial in Canberra, she decided to help the men. She gave them an ammunition box filled with a few dozen glass plates. Her husband, evidently, had kept some of the Thuillier plates that he had developed in 1988. “Pour les Australiens,” she said. She then suggested that other Thuillier plates might be hidden in an empty farmhouse jointly owned by Louis and Antoinette’s grandchildren—the same farmhouse where the portraits had been shot. After days of negotiations, Christian Thuillier, Louis and Antoinette’s grandson, led the visitors to an attic. Inside three chests were four thousand glass plates. Most of the Thuillier images had been perfectly preserved.

Coulthart persuaded the chairman of his television network, the Australian billionaire Kerry Stokes, to buy the entire collection from the Thuilliers, and to digitize the images. Stokes has since shipped the plates to Western Australia, where most are currently housed. (In 2016, a selection of the digitized photographs appeared in Coulthart’s “The Lost Tommies,” a large-format book.)

The digitized images are astonishingly clear: you can read the date of a newspaper folded on a lap. “What I found so fascinating was that we were seeing these images in a quality that they never saw in the First World War,” Coulthart told me. The portraits are also pulsing with life. The soldiers laugh, frown, flirt. They bounce local children on their knees, they pet dogs, they wear silly costumes, they sit on horses. In some photographs, the men have dirt on their boots and faces. It’s as if they’d left a trench only a minute before the shoot.

Coulthart and his wife, Kerrie Douglass, identified some of the photographs’ subjects by decoding visual clues, such as regimental-cap badges, and by cross-referencing the images with other sources. But, despite a continuing effort to identify the soldiers in the collection—a Facebook page allows users to search the images—many of the men remain unknown, including the few dozen American infantrymen. Almost every soldier of color in the archive remains unidentified, as are most of the thousands of Brits in the collection—including the majority of the Sixth. Although Kerry Stokes would like to find a permanent home in the U.K. for the fifteen hundred glass plates of British soldiers, no museum has offered to acquire them.

Looking at the portraits of the Sixth with the benefit of hindsight, one feels an awful foreboding. In one photograph, Second Lieutenant Cuthbert Higgins gazes at the camera with movie-star intensity. He died just weeks later, on the first day of the Somme, in the futile attack described by Tempest. It is possible to name Higgins because his portrait appears in the “History.” But other men are harder to identify. A classmate of Tempest’s at Bradford Grammar School, Ken Bloomer, was a private in Higgins’s company, and died next to him on the same afternoon. I have seen a portrait of Bloomer in the school’s archives, but if his childlike face is in the group photographs I cannot find it. Louis Thuillier was distraught by his inability to know the plight of his subjects; to view the photographs from the distance of a century can imbue the viewer with a similar melancholy.

The foreboding is not only historical. By April, 1916, the men had experienced ghastly things, and also knew that worse awaited them. Several of the photographs of the Sixth show Second Lieutenant Walter (the Babe) Scales, who earned his nickname because of his boyish looks. In one image, Scales, who had won a Military Cross for his bravery at the front, wears a crooked grimace that suggests recent anguish. In perhaps the most affecting image of the collection, a fellow-officer, who is still unidentified, links arms with Scales to support him. (Scales was killed in January, 1918, when a Royal Flying Corps airplane in which he was flying as an observer collided with another, above the Somme Valley.) Looking through the photographs, I thought of Tempest observing the men at one of the concert-parties: “in all cases the occasional look of strain or gloom : which though only momentary, expressed more than the average onlooker knew.”