The entry of the United States into the Great War a century ago today was the beginning of a new role for the nation in world affairs. But it also closed the chapter on a remarkable volunteer effort by young Americans that left a lasting literary legacy.

Between the outbreak of the war in 1914 and the United States entry in 1917, thousands of young Americans joined an ambulance brigade that provided first aid to Allied soldiers on the front lines. Among their ranks were many who would become preeminent writers. For two in particular—John Dos Passos and Ernest Hemingway—the war would become one of their great themes.

When the Great War broke out, American expatriates living in France obtained ambulances and ferried the wounded from the front to the well-equipped American hospital in Paris. This initial group came to be known as the American Field Service. Their actions inspired Richard Norton, an American archeologist living in Paris at the time. He set about launching an ambulance service with volunteers from the United States. The socially well-connected Norton found ready support for his plan. French millionaire H. Herman Harjes wrote large checks, and the venture was named the Norton-Harjes Ambulance Corps.

The Norton-Harjes Ambulance Corps pursued drivers as if it were looking for candidates for membership in an elite men’s club rather than for service in a war zone. “A volunteer must be a man of good disposition possessed of self-control—in short, a gentleman,” said one recruitment letter. From a practical point of view, targeting the American elite for recruitment made sense: It also took money to join—recruits were expected to pay for their passage and expenses. Ivy League campuses, with their wealthy students, were choice picking grounds.

Up until the American entry into the war, wounded Allied soldiers counted on Americans to get them to medical aid. But once American soldiers arrived in Europe in 1917, the two main volunteer ambulance corps were closed down. In early September, Norton announced an end to his corps. “As gentlemen volunteers, you enlisted in this service,” he said. “As gentlemen volunteers, I bid you farewell.” From now on the American Expeditionary Forces would be in charge and the Red Cross assumed ambulance duties.

Present in the encampment when Norton delivered the news was 21-year-old John Dos Passos. He had been among the drivers who had worked that summer on the front in Verdun, a stretch of land less than four square miles in size where hundreds of thousand French and German soldiers had died and a greater number were wounded fighting over a few thousand yards of blood-soaked soil.

In each trip back from the fighting, Dos Passos steered an ambulance loaded down with more wounded soldiers than it was meant to carry. Sometimes those soldiers with the more manageable injuries stood on the running boards or squeezed in on the front seat between Dos Passos and his assistant. “At every lurch, the wounded groaned horribly,” said Dos Passos.

The gruesomeness of what Dos Passos saw left him wordless. “I’m dying to write—but all my methods of doing things in the past merely disgust me now, all former methods are damned inadequate,” he wrote in his diary. “Horror is so piled on horror that there can be no more.”

When his ambulance corps was disbanded, he joined the Red Cross and was sent to Italy where he met a 19-year-old ambulance driver named Ernest Hemingway. Like Dos Passos Hemingway was certain that to miss the war was to be absent from the defining moment of his generation. “Believe me,” Hemingway wrote his sister before leaving for Europe, “I will go not because of any love of gold braid glory etc. but because I couldn’t face anybody after the war and not have been in it.”

A month after meeting Dos Passos, Hemingway was wounded by a mortar. Convalescing, he fell in love with his nurse, who would later inspire a fictional character, and with the close of the war he returned to the United States a war hero. Six years later the two writers were reunited in Paris and became fast friends and collaborators.

Their experience as ambulance drivers deeply affected them and would alter American literature. In the post-war years the books they wrote, such Hemingway’s A Farewell to Arms and Dos Passos’s Three Soldiers as well as his USA Trilogy, were innovative and important works, read by a nation coming to terms with the Great War. Many of those who would fight in the Second World War experienced the first cataclysmic war of the 20th century through the pages of these books.

To Hemingway and Dos Passos, the war had made traditional writing styles inadequate. Their generation needed its own voice, not one that imitated another from the past. Hemingway sought to describe the desolate postwar world with honest clarity, working like a jeweler in his quest to pen “the truest sentence that you know.” He believed that the perfect representation of an imperfect world alone was sufficient.

In contrast, Dos Passos wanted his writing to change the world. The war was no longer solely a cataclysmic horror that swept across Europe and maimed and killed 38 million people; rather, its unparalleled militarization was a harbinger of how society was robbing people of their individuality.

Dos Passos’s World War I novel Three Soldiers remains these many years later a highly readable articulation of opposition to war. But it is neglected while Hemingway’s A Farewell to Arms, embedded in the literary diet of English classes, presents war as an inevitability of life that tests one’s mettle and can bring the possibility of glorious love.

If the record of war in the 20th century is an indicator, Hemingway’s view prevailed while Dos Passos’s impassioned warning against war sadly fell on deaf ears.

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James McGrath Morris is the author of the just released The Ambulance Drivers: Hemingway, Dos Passos, and a Friendship Made and Lost in War, as well as four previous books, including the New York Times bestseller Eye on the Struggle: Ethel Payne, The First Lady of the Black Press, which was awarded the Benjamin Hooks National Book Prize, and the highly acclaimed Pulitzer: A Life in Politics, Print, and Power. He has appeared on NPR's All Things Considered, PBS's News Hour, and C-Span's Book TV. A former journalist, he was the founding editor of the monthly Biographer's Craft and has served as both the executive director and president of Biographers International Organization (BIO). Morris lives in Tesuque, New Mexico.