e are two years into the World War I centenary and historians have done their duty, providing ambitious books to explain a war that few people today understand in full. It is clear that the authors of the best of these works, most of them British historians, have been preparing for this anniversary and seem to sense that, among the general public at least, World War I is understandable only in the context of the war that came 25 years later. After all, we all know that Hitler fought in World War I, as did Churchill and De Gaulle. Shouldn’t we think of the two wars as one long conflict? After all, Churchill wrote in The Gathering Storm of “another Thirty Years War.”

Interpreting the Great War of 1914-18 as a bloody, horrific prelude to World War II, however, prevents us from understanding it as its combatants did. It was the first “modern war,” but also the last 19th-century one. The 1914 belligerents were much closer in time to those who fought the Civil War than we are today to those who fought World War II—in fact, they were almost as close to the Civil War as we are to Vietnam. For America, it would mark the last significant use of state-specific military units, such as the 129th Field Artillery of the Missouri National Guard, whose battery commanders included a young captain named Harry S. Truman. For Britain and Germany, it would mark the last great naval battle—the Battle of Jutland—of the Napoleonic era, involving only surface ships that relied on line-of-sight observation and visual signaling.

Moreover, seeing the Great War in terms of World War II doesn’t explain why the world’s most powerful empires began fighting in the first place—and kept fighting until exhaustion and revolution overwhelmed enough of them to call it off. A host of books published in the last three years have clarified how and why the war began. Writing in these pages two years ago (“On the Slaughter Bench of History,” Summer 2014), Algis Valiunas surveyed some of the best of these works, like Cambridge historian Christopher Clark’s, The Sleepwalkers, which examines the European history that led to war. The statesmen and generals, Clark writes, “walked towards danger in watchful, calculated steps.” How, then, were they sleepwalking? Valiunas answers, “The truly prudent captain or statesman is the rarest of political men—almost an impossibility. The man who craves power or empire for himself or for his country more commonly finds his capacity for prudential reasoning overpowered by his consuming appetite. The man of action often acts as in a daze, or in a dream.” And so the Great Powers went to war.

But it is even harder to understand why they kept fighting. A war that generals and politicians on either side claimed would be over in weeks or months had, by the end of 1915, claimed hundreds of thousands of casualties—sometimes in a single engagement, like the First Battle of the Marne, where a half-million soldiers were killed or wounded in a single week. The first 17 months of the war showed European powers how modern, industrialized warfare could efficiently annihilate people and devour resources. Why, knowing this, did they not desist?

According to Keith Jeffery’s 1916: A Global History, the empires kept fighting after 1915 because they could no longer afford to lose, or even to reach a negotiated peace. Their staggering losses had mounted so quickly, and their peoples sustained such a massive shock, that the very existence of the imperial regimes required complete victory. Jeffery, who was a historian at Queen’s University in Belfast until his death this past February, insists that global empires were necessary and sufficient for a global war, not just a war in western Europe. By early 1916 there were few places on the map not transformed by fighting. Jeffery’s approach is meant in part to be corrective: for America and much of western Europe, the history of the war, “has quite literally got stuck in the mud and trenches of the Western Front, in that blood-soaked frontier zone across the Low Countries and north-eastern France, which for centuries has been fought over by competing armies.”

Indeed, the most famous images and scenes of World War I come from the Western Front. We’ve all seen photos of the mounted German lancers in gasmasks, the smoldering no-man’s-land between enemy lines punctuated by a few splintered tree trunks. We think of the infantry charges over the top, the men in their steel helmets cut down by machine guns or blasted apart by artillery. We know the names of all these places: Ypres, the Marne, Passchendaele, the Somme, Verdun. We celebrate the valor of the soldiers and scoff at the incompetence of the generals—the “lions led by donkeys,” in historian Alan Clark’s famous phrase—and marvel that an entire generation was lost in futility and violence, a symbol of the insanity of industrial-scale attritional warfare.

This picture is not false, but neither is it complete. Jeffery marches through 1916 month by month, each chapter devoted to “emblematic events,” which serve “as hooks for a series of reflections” that chart “the astonishing range, variety and interconnectedness of the wartime experience.” He begins at Gallipoli, where the British pulled off an historic retreat in January, and concludes in Petrograd with the murder in December of Grigorii Rasputin, the weird "monk" whom the Romanovs took into their confidence. In between, Jeffery analyzes much else, including the famous battle of the Somme and the barely recalled revolt of indigenous Uzbek and Kyrgyz populations against Russian conscription. Rebels attacked Russian settlements, massacring thousands, and in response Russian imperial troops, along with bands of Russian settler vigilantes, inflicted a terrible retribution, massacring tens of thousands, and forcing a million or more Kyrgyz to flee over the mountains into Chinese Turkestan.

The episode connects directly to the generals and statesmen making war in Europe. The German attack on Verdun in February 1916 prompted urgent appeals by the English and French. They needed Czar Nicholas II to launch an offensive in the east, which he did a few months later at Narocz (in modern-day Belarus). An eastern offensive had been planned to coincide with the Allies’ Somme offensive that summer, but Verdun forced Russia into a hasty, ill-conceived affair that sapped the army and resulted in 100,000 in casualties. The result of that offensive, though, was the replacement of General Nikolai Ivanov, who had a nervous breakdown, with Alexi Brusilov as overall commander of the armies in his sector. What followed was the Brusilov Offensive, a stroke of tactical brilliance that would at once destroy the Hapsburg war machine but, disastrously for Russia, draw Romania into the war on the Allied side. What should have been a triumph for the Czarist regime was, instead, a turning point. By September both sides had suffered more than a million casualties, and the Russian Empire, sapped of its ability to wage war, was set on an irreversible path to revolution.

The losses incurred from the Brusilov Offensive necessitated the forced conscription that prompted the Uzbek and Kyrgyz revolt. Jeffery traces such domino effects across the globe, taking care to show how battlefield decisions on the Western Front mobilized entire populations a continent away. The displacements and upheavals of the Eastern Front, in areas that were home mostly to colonized peoples, “encouraged elements of the population to challenge the imperial status quo and assert a right to ‘national self determination.’” In many ways, the static nature of the Western Front spared the civilians there from the chaos and misery that mobile mass armies brought down on populations in the east, and in turn spared the regimes in London and Paris the fate of the Central Powers. Jeffery notes that although the engaged troops were far more numerous in the west, displaced civilians were more numerous in the east. By 1917, the total number of displaced people in the Russian empire was likely more than 6 million. It was, as the title of Peter Gatrell’s 1999 study of WWI refugees in Russia aptly puts it, A Whole Empire Walking.

Jeffery isn’t the only historian in recent years to identify 1916 as the war’s pivotal year. Adam Tooze, a former Cambridge historian now at Columbia, begins The Deluge, his acclaimed 2014 study of the war and its reshaping of the global economic order, in 1916. That was the year the Entente turned to the United States to finance its war effort, a development whose importance cannot be overstated. Before the war, London and Paris were the world’s financial capitals. By 1915, that role had shifted to Wall Street. As Tooze put it: 

The most powerful states of Europe were now borrowing from private citizens in the United States and anyone else who would provide credit. Lending of this kind, by private investors in one rich country to the governments of other rich developed countries, in a currency not controlled by the government borrower, was unlike anything seen in the heyday of late Victorian globalization.

In 1916, this sort of lending would reach a new level. The Allies needed a way to finance the gigantic Somme offensive planned for that summer, so they turned to the J.P. Morgan investment bank, which in the course of the Somme campaign would finance 45% of British war spending—more than a billion dollars of which was spent in America on behalf of the British government. Tooze puts this in context, noting that, “In 1916 the bank’s purchasing office was responsible for Entente procurement contracts valued in excess of the entire export trade of the United States in the years before the war.”

Politically, this level of financing turned the war into a transatlantic affair a full year before America formally entered on the side of the Allies: “Through the private business contacts of J.P. Morgan, supported by the business and political elite of the American Northeast, the Entente was carrying out a mobilization of a large part of the US economy, entirely without the say-so of the Wilson administration.” By the time Wilson acquiesced, American industry had already tipped the balance of the war in favor of the Entente.  

Tooze’s larger argument is that because America was the only country the war strengthened rather than weakened, she had a responsibility to manage its aftermath. The Deluge contends that the dramatic collapse of the German, Austro-Hungarian, Russian, and Ottoman empires, and attenuation of the British and the French ones, made it possible for America to restore Europe’s economy by canceling inter-Allied debt, which it refused to do. During the 1920s, that decision triggered a dangerous cycle of debt repayment from Britain and France, financed by war reparations from Germany that were, in turn, made possible by American loans. Huge amounts of money were being recycled, laundered through unstable new international mechanisms that would prove inadequate and eventually calamitous. There is a reason Tooze ends his narrative in 1931.

Few emperors and statesmen knew in 1916 that they had crossed a Rubicon—that the war had sounded a great bell, which could never be un-rung. Yet sometimes the poets know. William Butler Yeats completed “Easter, 1916” in September of that year, but owing to its political nature—a rarity for Yeats—the poem was privately printed and distributed the following year. It would not be published until 1921.

Jeffery, who was himself a member of the Royal Irish Academy and author of several books on the history of modern Ireland, writes that Yeats knew the Easter Rising, and in particular the execution of its leaders by firing squad, “caused a seismic shift in attitudes after which nothing could ever be the same again.” In the poem’s final lines, Yeats invokes the names of the executed rebel leaders—“MacDonagh and MacBride / And Connolly and Pearse”—names that Irish citizens would later invoke alongside casualty lists from the Western Front. The Rising, Jeffery argues, “can only be properly understood in the context of the Great War…. As surely as Verdun or the Somme, Dublin in 1916 was a First World War battlefield.” Perhaps that is part of what Yeats is getting at in his poem’s haunting refrain. He could have been writing about the whole world—a world “changed, changed utterly: A terrible beauty is born.”