In 1917, the second largest British city, after London, wasn’t in Britain at all. It was the British Expeditionary Force (BEF) in France. The same could be said of the French Army, and the German. Each force on Europe’s Western Front was a metropolis requiring not just soldiers, guns, and ammunition, but every necessity of life, from toothbrushes to rubber stamps.

To service these vast cities of war, nations built new rail networks, roads, hospitals, food processing facilities, warehouses, and even brothels. In 1916, the British were supplying their troops with 2,925 cubic feet of tobacco a day – the volume of a semitrailer. Building just one mile of trenches required 900 miles of barbed wire, 6,000,000 sandbags, 1,000,000 cubic feet of timber, and 360,000 square feet of corrugated iron.

This is to say nothing of the munitions that were needed: At the height of the conflict, the BEF was going through 70,000 grenades a day.

Nothing is bigger than a world at war. One hundred years ago, a monster took hold of Europe, rending it in ways that no other war over thousands of years of history had ever done.

For most Europeans, the Great War, which began innocuously with a Serbian nationalist’s assassination of Archduke Franz Ferdinand of Austria in June 1914, represented the end of splendor and the beginning of mediocrity. “This is going to be our evil inheritance,” wrote Italian soldier Paolo Monelli.

For most Americans, however, the war seems a minor affair that pales in significance to the Civil War, World War II, and Vietnam. This is ironic given how the war shaped what came to be known as the American Century. So many of the triumphs and tragedies America has faced since its emergence as a superpower have their origin in World War I. It is not preposterous to imagine that, without the Great War, there would have been no Joe McCarthy, no Korea, no Vietnam, no NATO, and no moon landing, for all were connected to the new world order that emerged after 1918.

The war brought the collapse of three empires, hastened the demise of a fourth, and, it could be argued, gave rise to a fifth – the United States. It shaped the boundaries of many of today’s nations, and gave birth to tensions that still divide them. Even today’s bitter conflict over the Palestinian territories was shaped by decisions made during World War I.

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Europeans still call it the “Great War.” “Great” meant big, since no previous war approached it in scope. There were more soldiers, more belligerent nations, more weapons, bigger battlefields, more deaths than ever before. Bigness was also meant geographically. Naval battles occurred in the North Atlantic and South Pacific. Soldiers were killed on three continents. German shells, Turkish mines, Austrian machine guns, Greek pathogens, and even African lions killed English soldiers.

“Great” also referred to purpose. This was the “war to end all wars,” the “war to make the world safe for democracy.” For the British, it was an existential conflict.

“I do not believe any nation ever entered into a great controversy ... with a clearer conscience and a stronger conviction,” Prime Minister Herbert Asquith told the House of Commons on Aug. 6, 1914, “that it is fighting, not for aggression, not for the maintenance even of its own selfish interest, but ... in defence of principles ... vital to the civilization of the world.”

In September 1914, the chancellor of the Exchequer, David Lloyd George, reminded his audience at the Queen’s Hall of the “great everlasting things that matter for a nation; the great peaks we had forgotten ... the great pinnacle of Sacrifice pointing like a rugged finger to Heaven.”

Similar words were spoken in every combatant nation. War produced an excess of noble purpose.

Nor was this simply the stuff of propagandists. Nearly everyone, on all sides, yearned for sublime meaning. Danish soldier Kresten Andresen fought for Germany simply because his homeland had been absorbed by Prussia in the war of 1864. He harbored no love for his political masters, but still welcomed the chance to fight for them.

“Go to war not for the sake of goods or gold, not for your homeland or for honor, nor to seek the death of your enemies, but to strengthen your character, to strengthen it in power and will, in habits, custom and earnestness,” he wrote. “That is why I want to go to war.”

In contrast, the poet Rupert Brooke sought the purest patriotic sacrifice:

If I should die, think only this of me:

That there’s some corner of a foreign field

That is forever England.

“It is our great privilege,” a British soldier told his parents, “to save the traditions of all centuries behind us. It’s a grand opportunity, and ... if we fail we shall curse ourselves in bitterness ... and our children will despise our memory.”

The war came at a unique moment when it was very easy to get men to fight but also very easy to kill them. Quaint notions of heroism disguised a thoroughly modern war. Yet when old values collided with modern machinery, the former stood no chance. Tradition implied romantic cavalry charges, noble death, a single merciful bullet through the heart.

Before the war, English schoolboys read H.A. Vachell’s “The Hill,” which glorified the opportunity “[t]o die young, clean, ardent; to die swiftly, in perfect health; to die saving others from death ... to die and to carry with you into the fuller ampler life beyond, untainted hopes and aspirations, unembittered memories, all the freshness and gladness of May.”

When those boys turned into men, the anticipation of glorious death provided thin armor against cruel modernity. This was a factory war of thunderous artillery, of bodies lacerated by machine gun fire, of men drowning in mud.

Edward Mousley, a cavalryman from New Zealand, felt acutely the consequences of soldiering on the hinge of two ages. He went off to war with his best friend, a stallion he’d owned since boyhood. Caught in the demoralizing siege at Kut al Amara, where British troops were routed by Turkish and German forces in what is now Iraq, he was forced to eat his horse.

The war was a monster that fed on the fodder of humanity. Its nature was determined by the state of technology: Machine guns and heavy artillery, not incompetent command, forced men into trenches. Commanders desperately sought a way around the cruel arithmetic of attrition, but could not find one. Heavily laden men trudged forth from the relative safety of their trenches into the merciless storm of gunfire because that, in truth, was the only way to fight this war.

Nowhere were the horrors of trench warfare more on display than in the Somme Offensive, the clash between British and French troops and their German rivals along the Somme River in northern France in 1916. It was intended to be the last decisive battle of the war.

For days, British and French troops had bombarded German positions with artillery – more than 1.7 million shells – to destroy their trenches and hedges of barbed wire. Then, on July 1, the men from 11 British divisions emerged from their earthen fortifications to advance slowly toward the German lines. They were unaware that the Germans had created deep dugouts for their men that had protected them from the artillery assault.

Like a vast engine of death, the German machine guns came alive and carnage commenced. By the end of the first day, the British had suffered more than 57,000 casualties, of whom 19,000 were dead. It was the worst military disaster in British history and one of the bloodiest days in any war.

Yet the British attacked again the next day, and the next. The Somme would not end until almost four months later, on Nov. 13, 1916. By that time, the British had suffered 420,000 casualties, the French 195,000, and the Germans nearly 600,000. Suffering was punctuated by the day-to-day indignities of trench life – the mud that rotted feet, the legions of rats, the putrefying corpses everywhere.

The monster of war would ravage for as long as nations could feed it. This was total war, a test of national commitment, organization, and morale. Victory would go to the side able to mobilize a huge army and keep it supplied. This meant that endurance and improvisation counted more than bravery.

In Germany, there were 837 registered meat substitutes in 1918 (including ones made of nuts and animal organs), but hardly any meat; 511 registered coffee substitutes (roasted acorns, barley with coal tar), but no coffee. The British commander Douglas Haig became convinced that the German soldiers’ morale would crack when their sausage ceased to contain meat. Whether coincidence or not, that proved true.

As the war consumed men, it also devoured romance. Monelli, a writer and member of Italy’s elite Alpini mountain infantry, went to war in search of glory, but ended up cursing “those mysterious gods who spin the threads of our fate.”

The war demonstrated that man was not the agent of his own destiny, that the machines he had created were beyond his control. Fearing the implications of a modern leviathan, Lord Lansdowne in late 1917 urged the British government to press for a negotiated peace. To push on to victory would, he maintained, “spell ruin to the civilized world.” All that had made Britain great would be destroyed; “we are slowly killing off the best of the male population of these islands.”

But Lansdowne was scorned, because too much had already been invested. Only victory, it seemed, would make the massive sacrifice worthwhile.

Victory, however, carried an inconceivable price. It came, unexpectedly, in 1918. The Germans, already triumphant on the Eastern Front, massed their armies in the West for one last, desperate push. Their spring offensive was frighteningly effective, but, when the surge slowed in June, German morale collapsed.

The British and French, now fortified by American troops, counterattacked, forcing the Germans into headlong retreat. Fearing invasion, they sued for peace. It is, nevertheless, important to remember that it was the nation itself, not the army, that had been defeated. At war’s end, German soldiers were still entrenched in Belgium and France. That footnote to defeat would prove important when Germans remembered the war.

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When the guns fell silent, the battle of meaning began. Those on the winning side at first found value in victory. “We may have been naïve...,” wrote English nurse Beryl Hutchinson, “but we all had the feeling that we really were keeping the world fit to live in, that our many sacrifices had been worthwhile.”

That confidence, however, depended upon the emergence of a stable world after 1918. As it turned out, what transpired was far from satisfactory. Soldiers came home to mass unemployment, an insult to their sacrifice. Outrage was directed at fat profiteers who had feasted on suffering. The rancorous and cynical peace conference at Versailles undermined confidence in the emergence of a better world. Some hope was invested in collective security through a League of Nations, but that disappeared when America retreated into isolation.

The dam holding back doubt burst around the time of the Wall Street crash. By 1929, most people agreed that the war fought to preserve something great had produced instead something sordid. With romance stripped away, the war began to seem like what it was: a cynical clash of empires.

Brooke’s vision of noble death in a foreign field gave way to what another great British war poet, Wilfred Owen, called “the old lie.” Henceforth, politics would poison interpretations of the war’s meaning. Nowhere was this more the case than in Germany, where fascists blamed Jews for the humiliating capitulation that occurred while German forces still occupied enemy soil. Hitler would make great use of this “stab in the back.”

In Russia, bitter memories of the “capitalist war” were used to sustain the Marxist revolution and camouflage military defeat. Elsewhere, class antagonism spread like a contagion. In Britain, workers concluded that the incompetent “donkeys” of the military high command had squandered the lives of ordinary soldiers in a selfish attempt to preserve privilege.

In the 1924 election, the Labour Party capitalized on this sense of betrayal. Posters juxtaposed two highly emotive images: “Yesterday the Trenches, Today Unemployed.” The contrast questioned the war’s worth and suggested that workers still had a debt to collect.

This ugly battle between right and left over the war’s legacy continues to this day. A few months ago, Michael Gove, the British government’s Conservative education secretary, clashed with Sir Tony Robinson, star of “Blackadder Goes Forth,” the 1989 satirical series about World War I, over how the war should be remembered. Mr. Gove feels that generations of British schoolchildren have had their patriotism poisoned by the war poets, the “Blackadder” series, and the BBC.

The war was not futile, but neither was it particularly heroic. The right side did win, but motives were far from pure. The war was indeed a capitalist conflict: Britain and France felt threatened by modern, ambitious, industrialized Germany. The prospect of German domination of continental markets was particularly frightening for Britain, since the decline of its empire had rendered her increasingly dependent upon European trade.

On the eve of war, when intervention was still being debated, Foreign Office official Sir Eyre Crowe provided a devastating analysis of Britain’s predicament. The country had to fight, he explained, because if it stood aloof, it would be despised and dominated by whichever side won. The British did not particularly like the French or Russians, but needed allies to halt German hegemony. War was the continuation of economics by other means.

Britain at first thought it could fight with minimum commitment. The French and Russians would do the dying, financed by British banks. This was called, appropriately, “business as usual.”

That strategy quickly proved untenable, however. Within weeks, Britain was forced to make a massive commitment to the war, in soldiers, munitions, and money. The conflict that had originally seemed like economic opportunity threatened instead financial disaster. Britain began the war a creditor nation. It ended it with a mountain of debt owed to the US.

Backbone industries – steel, coal, shipbuilding, and textiles – were all exhausted by war. There was no money for the essential modernization necessary to compete with America, Japan, and, eventually, Germany.

Nor did the empire provide the economic security it once had. It expanded after the Versailles settlement, but was now more liability than asset. Victory, in other words, looked a lot like defeat.

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In November 1918, every European nation teetered on the brink of ruin. The beneficiary of this collapse was the US. America emerged from the war enormously wealthy and relatively unscathed. Some 116,000 “doughboys” died, but less than half fell on the battlefield. The biggest killer was Spanish influenza. The Americans had avoided the war for as long as possible, judging it a peculiarly European disease. Afterward, they did their best to forget it.

Yet it is fair to say that the Great War made the US what it is today. America’s destiny was determined by the collapse of three empires and by the decline of another.

First, the disintegration of the Habsburg Empire resulted in dangerous instability across Central Europe, from Vienna to Bucharest. Hitler, then Stalin, would exploit that instability. It would surface again in the 1990s with ethnic violence in the former Yugoslavia eventually requiring NATO intervention. The region is quiet now, but peace in the Balkans will always be fragile.

The collapse of the Russian Empire led to the emergence of a communist Soviet Union and, eventually, to the cold war, the defining conflict of the baby boomer generation. Nor did the Russian problem disappear with the collapse of the Soviet Union. Russian President Vladimir Putin’s covetous forays into Ukraine look a lot like an attempt to turn the clock back to before 1914. A Putin aide recently confessed that the Russian president also wants Belarus, Georgia, and Finland – the czar’s old possessions.

The demise of the Ottoman Empire also shaped American destiny. It had shrunk significantly by 1914, but still included portions of present-day Turkey, Syria, Jordan, Saudi Arabia, Iraq, and the Palestinian territories. The region has never recovered the stability that the Ottoman emperors, in their heyday, once imposed.

Regional tensions have been exacerbated by the West’s thirst for oil. Further instability was caused by the contradictory deals Britain struck with Jews and Palestinians. In the heat of World War I, those deals seemed to make sense, but short-term gain led to a century of bloody conflict that America has not been able to ignore.

Finally, the slow demise of the British Empire offered opportunities for Americans, but also brought burdens. When the center of world finance shifted from London to New York, the nature of Anglo-American relations permanently changed. In 1931, conditions imposed by New York bankers contributed to the collapse of the British government. That would have been unthinkable before 1914.

With power, however, came responsibility. As the British (and French) retreated from empire, they bequeathed problems that would later plague Americans – in the Middle East, Africa, and Southeast Asia. The Great War eventually forced America to adjust to the burdens of power, just as Britain had to do in the 19th century. This has meant that the US has occasionally felt the lament voiced by Crowe about Britain in 1914 – a strong nation surrounded by threats and desperately in need of friends.

Some historians like to see the two world wars as one conflict that lasted from 1914 to 1945. That is an intriguing notion, but not a particularly illuminating one. What can be said with certainty is that World War I was the main cause of World War II. The 1914-18 war sowed the seeds of German revanchism and also rendered the British and French too weakened to respond effectively to Hitler’s ambitions.

But it is also the case that the two wars together produced the longest period of peace in Europe since Roman times. Europe has grown tired of war. The byproduct of that exhaustion is the European Union, arguably the biggest economy in the world. The original architects created the EU with the aim of building a network of commerce so strong that war would become unthinkable.

The Great War was a modern conflict fought with romantic intent. Soldiers who marched off in search of glory soon discovered man’s prodigious capacity for industrialized slaughter. Horrors now familiar – killer submarines, chemical weapons, aerial bombardment – all have their origins in World War I.

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The wounds of that conflict – in Africa, the Middle East, and the Balkans – still fester. Pick almost any problem in international relations today and the links to 1914 can easily be traced. The war is best seen as a massive earthquake that permanently altered the social and political landscape of the world. Its aftershocks still rumble.

Gerard DeGroot is a professor of history at the University of St. Andrews and the author of five books on World War I.