The British military top brass told their men they were about to take part in the "the greatest battle in the history of the world". What they were about to experience, however, was a "bloody great balls-up" on an industrial scale.
For Rudyard Kipling, the most famous author of the age, the carnage at Loos on the Western Front in September 1915 plunged him into inner darkness. His only son, John, for whom he had written his best-loved poem, If, had been killed in the action just six weeks after his 18th birthday.
Last seen on the second day of the ill-fated attack, stumbling blindly through the mud, screaming in agony after an exploding shell had ripped his face apart, the failure to find John's remains fuelled the author's obsession that his son had survived. But it was not to be. Kipling eventually came to accept John's fate. And despite a grief-stricken crusade to find them, the remains of his "dear old boy" were not officially "discovered" until 1992. Yet there are those who believe that the body interned in a grave bearing his name at plot seven, row D of St Mary's Advanced Dressing Station Cemetery, near Loos, are not those of the author's son.
John's death rocked his father's belief in the British military elite, particularly General Douglas Haig, who went on to lead the war effort as a result of the battle. Loos was also to transform the way Britain's war dead were remembered. But it did nothing to dent Kipling's deep and passionate patriotism.
It was more than a year into the First World War when the Anglo-French forces, bogged down on the Western Front, sought to deliver a long-awaited breakthrough.
The British offensive boasted six divisions under the leadership of General Haig, and despite outnumbering the opposition by seven to one, the surrounding countryside bristled with German machine-gunners.
Perceiving - rightly - that he had insufficient artillery, Haig ordered his officers to deploy 140 tons of chlorine gas, the first time chemical weapons were used in the war. After a four-day bombardment, in which 250,000 shells were fired, British troops took Loos, only to lose it the following day as the Germans launched a counter-offensive, driving them back to their original positions.
As the British fought back, they advanced without artillery support, and were cut down in their thousands by a blizzard of German machine-gun fire.
The reserves arrived too late and communications lines failed. The wind changed direction, resulting in the gassing of thousands of British troops.
By the end of the week there were some 75,000 casualties, two-thirds of them British. Among the dead was the poet Charles Sorley and the young brother of the future Queen Mother, Fergus Bowes-Lyon. The "balls-up" was recorded for posterity by one survivor, a young Robert Graves, in his autobiographical Gooodbye To All That.
Like many of those who fell alongside him, Loos was John Kipling's first taste of war. He joined the fray two days into the battle as part of a reinforcement contingent of Irish Guards.
John had been desperate to join up, and even before the war, the military had been his longed-for destiny. While Rudyard might have chosen the Navy, young John wanted to be a soldier. But his eyesight, like his father's, was appalling. His was so poor that he was unable to read the second letter on the chart, despite his thick glasses.
"John was extremely keen to join up. Like pretty much everyone else he thought it would be a short war and wanted to play his part," said Michael Smith, a vice-president of the Kipling Society. "He went at the beginning to try and enlist on his own, but was rejected. Later he tried again, this time accompanied by his father, but again he was rejected."
It was time to pull some strings. His father was at the height of his celebrity. The world's youngest Nobel literary laureate, his was the authentic voice of empire, whose work beat the drum for the jingoistic spirit of the times.
And the writer's military connections were at the very highest level. Rudyard had been life-long friends with Lord Roberts, commander-in-chief of the British Army, and colonel of the Irish Guards.
John was accepted into the regiment and began his training as an officer cadet at Warley Barracks. In many ways the young Kipling cut an undistinguished figure. He struggled to make it into Wellington School, relying on the services of a crammer to pass the entrance exam.
He was fond of cricket but not reading. Although he was brought up listening to his father reading the Just So stories at bedtime, he never willingly picked up a book.
The young Kipling found himself embarrassed by his father's celebrity, particularly aged 12 after the publication of If - the poem dedicated to him.
The close relationship between father and son grew out of Kipling's own unhappy childhood experiences. According to Tonie Holt, author of My Boy Jack? The Search for Kipling's Only Son, the youngster exerted a profound effect during his short life. "He was a sparky little guy and has been virtually ignored by everybody in the story of Rudyard Kipling," he said.
After the loss of his older sister, Josephine, who died during a violent Atlantic crossing in 1899 which nearly claimed Rudyard Kipling himself, John became the centre of his father's attentions. "He was besotted with his son," says Mr Holt. "They would correspond regularly, his 'dear dada' was always giving him advice on what to do."
When he finally joined up John lived the life of a typical upper middle-class subaltern. His military commitments scarcely interfered with his busy social life. Although he was not considered a playboy, young Kipling was a regular visitor to London night clubs, and loved the country house parties at the family home, Batemans in Sussex. Like his father, he was a motoring enthusiast, owned his own motor bike, and enjoyed mixing with the cream of Victorian society.
His parents remained realistic about his survival chances. After his mother, Carrie, waved him off, she wrote in her diary: "There is nothing else to do. The world must be saved from the German ... One can't let one's friends and neighbours' sons be killed in order to save us and our son."
Yet when it came to it, John's death was a hammer blow to Kipling, who was working as a war reporter in France at the time. The news was delivered by his friend, the Tory leader Andrew Bonar Law, and the author cried a "curse like the cry of a dying man".
According to Michael Smith: "The Kiplings were devastated. They thought with any luck he may have been kidnapped - even dropping leaflets over the frontline by plane seeking information about his whereabouts."
Tonie Holt described how the author carried out hundreds of interviews with his late son's comrades, building up a detailed picture of his last moments. He believes that it is through this research that the claim that John's remains are in the Commonwealth War Graves Commission can be disproved. Not only is the rank on the gravestone wrong - Kipling's promotion to Lieutenant had yet to be announced in the London Gazette - but the remains were found some two miles from where he fell, at a feature called Chalk-Pit Wood.
The devastated father threw himself into his work, becoming a prominent member of the commission. He took part in the creation of the pristine rows of Portland stone graveyards, which now honour Britain's fallen, selecting the Biblical phrase "Their Name Liveth For Evermore" as a fitting epitaph.
Yet his career was by now in decline, and his work failed to strike a chord with a generation traumatised by the memory of the slaughter of the trenches.
Rudyard Kipling remained unbowed in his political views and remained a vehement opponent of German rearmament. His love of the military was also undimmed - he wrote a regimental history of the Irish Guards, considered to be one of the finest ever and which contains a heartbreakingly brief description of his own son's death.
He was never able to write directly about John's loss. My Boy Jack is about a sailor - but still a thinly disguised poem about regret and mourning. Shadows of guilt have also been detected in his later work. "If any question why we died/ Tell them, because our fathers lied" is thought to refer to his role in helping his son to bypass the military eyesight rules.
Rudyard Kipling lived until January 1936. But father and son live on in the nation's consciousness. If remains Britain's favourite poem.