This story appears in the March/April issue of National Geographic History magazine.

As simple as it may sound, the European world collapsed on itself like a matchstick castle in summer 1914. Britain, France, and Russia were bonded in a “Triple Entente,” while Germany, Austria-Hungary, and Italy were bound in a Triple Alliance. These conflicting alliances from previous wars pulled and tugged at the structure until it came tumbling down on June 28, when a Serbian nationalist killed the visiting Archduke Franz Ferdinand, heir apparent to the Austro-Hungarian throne. With nationalist elements threatening to pull its empire apart, Austria-Hungary struck back at Serbia, who then called on its ally Russia for aid. That inspired Germany to declare war on Russia and France, and Great Britain to respond with a declaration of war on Germany. Japan, allied with Britain, followed suit.

At first, the United States declared itself officially neutral, and President Woodrow Wilson counseled his fellow countrymen: “The United States must be neutral in fact as well as in name during these days that try men’s souls. We must be impartial in thought as well as in action, must put a curb upon our sentiments...” Sound and high-minded advice that would be hard for everyone, including Wilson, to follow.

View Images Point of Entry To encourage enlistment in the American Expeditionary Forces (AEF) shown here in Lorraine, France, the U.S. government commissioned artwork, like the iconic Uncle Sam recruiting poster (above). By war’s end the AEF would number more than one million. Photograph by Bettman/Getty Images

Twentieth-Century Horrors

No one could have foreseen the savagery unleashed by the world’s first industrialized war, where the efficiency of modern killing machines surpassed anything imagined in past European conflicts. On the battlefield, 19th-century tactics soon proved useless against 20th-century weapons. Terrorizing the ground, machine guns had a firepower that equaled 80 rifles. Advances in artillery rained down explosives on soldiers in the trenches. Armored cars and tanks first rolled their way into battle in World War I. Chemical warfare, in the form of chlorine, mustard gas, and phosgene, poisoned hundreds of thousands of soldiers.

View Images Victory Medals The United States issued these medals to members of the U.S. military who served in World

War I. The back of the medal lists the Allied countries, while the front depicts a figure of the winged victory. Photograph by NGS Image/Dagli Orti/Aurimages

Advances in technology led to battles taking place almost anywhere on Earth. Devastation threatened from above and below, with dirigibles prowling the skies and submarines prowling the seas. Observation balloons were used for gathering intelligence, and zeppelins were used in bombing raids. World War I was the first major war to be fought in the air; British, French, and German flying aces engaged in famous dogfights over Europe. In the seas, Germans held the advantage: Their U-boats were state of the art, a submarine more advanced than any other nation’s. A U-boat could carry 35 men and 12 torpedoes and travel underwater for two hours at a time.

By late August 1914, the war was being fought on two fronts—eastern and western—and Germany was winning on both. It had destroyed the Russian Second Army at the Battle of Tannenberg, and it had invaded Belgium and northeastern France. Its forces were within 30 miles of Paris when a combined allied force of French and British halted the German juggernaut and pushed it back beyond the Marne River. Some 6,000 French reserve troops had made it to the front, thanks to an army of Parisian taxi drivers who ferried them there.

As fall moved toward winter, both sides literally dug in, creating an elaborate system of trenches across northeastern France. The war that had moved with such lightning speed in its early months became bogged down along the western front into a Dantean hell of trench warfare that would last for years.

No young man who had marched eagerly off to the Great War could have anticipated that subterranean, surreal world, where a soldier could wait month after month in wet fetid ground, in heat and cold, staring at the enemy across a no-man’s-land, knowing that incoming shellfire or poison gas or a sniper’s bullet could take him or a buddy out at any time. Lice, filth, boredom, and the noxious stench of rotting flesh, sweat, cordite, and human waste plagued the men as much as disease and death. Even in the first year of the war, some men began to suffer from a strange bundle of symptoms that left them incapacitated with confusion, fatigue, tremors, nightmares, and impaired sight and hearing. The men named it themselves—they called it shell shock.

Europe 1914 Before the war, Europe was bound up in two alliances that would morph into Worl War I’s Allies and Central Powers: The Triple Entente—the United Kingdom, France, and Russia—and the Triple Alliance—Germany, Austria-Hungary, and Italy. After the war began, allegiances shifted: Italy, neutral at first, switched sides in 1915 and joined the Allies, as did Portugal and Romania in 1916 and Greece in 1917. The Ottoman Empire joined the Central Powers in October 1914, followed by Bulgaria in 1915. NG MAPS/ROSEMARY WARDLEY AND GREG UGIANSKY

Mounting Pressures

By the war’s second year, the conflict had engulfed virtually all the world, from major powers to minor principalities. Japan had attacked German-held territories in China and the Pacific, while Russia had declared war on the fading Ottoman Empire, leading British, French, and Anzac forces to attempt a disastrous invasion of Turkey through the Dardanelles and up the Gallipoli Peninsula.

View Images The Last Emperor Ruler of the German empire, Kaiser Wilhelm II lost much of his political influence during World War I as his generals gained power. Photograph by Alamy/ACI

America watched the events with a wary eye, still determined to remain neutral. Vocal factions, including suffragettes, prohibitionists, and Wobblies—members of the Industrial Workers of the World—had their own political agendas that they feared would be derailed if war diverted the public’s energies and attention. And President Wilson continued to preach neutrality, even as he nurtured a deep-seated love of things British, particularly their laws and literature (he was an especially ardent admirer of Wordsworth). A shared language and traditions inclined many Americans toward their close cousins across the Atlantic, but not all. German Americans and Irish Americans had no sympathy for the British or their cause, particularly in the first year of the war.

In the second year, though, things began to sway the general populace toward war. On May 7, the British Cunard liner Lusitania was torpedoed by a German U-boat off the Irish coast, and almost 1,200 passengers were lost, including 128 Americans. The Germans had warned that they would sink enemy vessels on sight, and in truth, the Lusitania was carrying munitions. Still, the U.S. public was outraged by the attack, and British propaganda fanned the flames.

View Images Sinking of the Lusitania Survivors escape the sinking Lusitania, the British liner attacked by a German U-boat near the Irish coast on May 7, 1915. Germany had warned that any ship in British waters was a potential target. Photograph by Leemage/Prisma Archivo

Britain’s ships had been blockading Germany for months, and its navy had cut the undersea cable that allowed its enemy to communicate easily with the Western world. That move, as much as any strategic battlefield maneuver, guaranteed Britain dominance in the war to win American hearts and minds. The British propaganda machine kept up a steady diet of stories detailing and hyperbolizing German atrocities against Belgian civilians. Meanwhile, orders from Britain and France for munitions and food were pouring into the United States, fueling a war boom that gave a healthy boost to household incomes.

Even as the U.S. unabashedly supplied France and Britain, and the old warrior Teddy Roo­sevelt pounded his fists for war (considering yet another run for president in 1916), Wilson officially clung to his position of neutrality. Through his unofficial ambassador to the belligerents, Col. Edward House, Wilson offered to mediate a peace between the warring parties. But he also sought congressional approval for a large military buildup in the fall of 1915.

War Rages On

America was far from prepared for a large-scale war, much less one overseas. In 1914, when the conflict began, the United States had less than 100,000 soldiers scattered across the world, with another 120,000 in the National Guard. When Wilson had taken office, the greatest external threat the country faced came from an unsettled revolution and unrest in Mexico. That situation remained unresolved, but more pressing was the problem of freedom of the seas.

View Images Badge of Honor Instituted in February 1918, the German government awarded the U-boat War Badge to crews in the Imperial Navy who had completed three war patrols. Photograph by Don Trojani, Bridgeman/ACI

Germany had warned that even neutral vessels in British waters “would be destroyed without it always being possible to warn the crews and passengers.” Since the war began, it had ramped up U-boat production to a frantic pace, and submarine stealth and precision had brought down several British ships and kept Wilson ever on guard and protesting. Not the strongest candidate for the position, the U.S. secretary of the Navy—political loyalist, pacifist, and small-town newspaperman Josephus Daniels—unfortunately had no experience with naval affairs, or boats for that matter. The more experienced assistant secretary, Franklin Delano Roosevelt, would later say: “The Secretary was told hundreds of times that in case we should need... to jump from 52,000 to even 70,000 to 150,0000 men in a month or two [it] would absolutely upset the existing organization.”

Yet that need would soon come as the European powers relentlessly continued the fight. Despite Wilson’s constant efforts to find an end to the “war to end wars,” none was in sight in 1916. All of Colonel House’s back-door negotiations had resulted in little and convinced him that each of the major belligerents wanted a total victory, not a brokered peace. Kaiser Wilhelm had determined to focus Germany’s might on crushing Britain through an unconditional submarine war against its merchant ships, and at the same time to “bleed France white” with a set-piece siege of the fortress Verdun, in northeastern France.

The Germans attacked Verdun in late February 1916, and for eight interminable, inhumane months, the siege dragged on. Always intent on perfecting its chemical warfare techniques, Germany had lobbed phosgene gas at the Allies during the siege; once in the lungs, the deadly compound turned to hydrochloric acid, burning men from the inside.

Meanwhile, the British had developed their own new weapon—the tank. And in early summer, during their tragic and misbegotten Somme offensive, tanks rolled onto the battlefield for the first time—to little avail. It took the British and French until November to advance seven miles along the northwestern front, and it cost them more than 620,000 casualties. By the time the French finally broke through at Verdun a month later, they had suffered close to 400,000 casualties and the Germans 350,000.

Neutral No More

While these grueling battles tore apart Europe in 1916, the United States suddenly had its own conflict to address much closer to home. The Mexican bandit general Pancho Villa was spoiling to engage with American forces, and to that end had attacked the small town of Columbus, New Mexico, in early March. Within days, Brig. Gen. John “Black Jack” Pershing was on the march, leading a punitive, and ultimately fruitless, expedition to track down Villa.

View Images Effective Explosives Grenades were not new technology, but the nature of trench warfare resulted in the development of different types of bombs, such as the double-cylinder model. Photograph by BikeMech/Getty Images

Soon, much of the existing U.S. Army and tens of thousands of National Guardsmen had made their way to the Southwest, leaving the Atlantic coastline open to German attack or sabotage. It came at the end of July 1916. Black Tom Island, a major depot off the coast of Jersey City, New Jersey, held much of the munitions being sold by the United States to the British. Covert German agents engineered an explosion on the island, in what some historians claim is the first act of terrorism on U.S. soil by a foreign power. Citizens in Manhattan and Jersey City saw the blast close up, but people as far away as Philadelphia felt and heard it, too. Even the Statue of Liberty was damaged: Visitor access to her torch closed after the explosion and has remained off-limits ever since.

Two months earlier, Wilson had been in New York to address the League to Enforce Peace, assuring its members that “[t]here is nothing that the United States wants for itself that any other nation has,” and that its “interest is only in peace and its future guarantees.” Wilson continued that message into the fall of 1916, as he fought a close presidential campaign against Republican Charles Hughes. Wilson won, but barely, with 277 electoral votes to Hughes’s 254.

View Images Mexican revolutionary Pancho Villa aroused the ire of the United States when he executed some 17 U.S. citizens in Chihuahua, Mexico, in January 1916. Two months later he attacked Columbus, New Mexico, prompting U.S. President Wilson to send the army to track him down. Photograph by Granger/Album

In March 1917 Wilson’s steadfast neutrality was pushed beyond even his limits, when the British intercepted and deciphered a telegram sent by German foreign minister Arthur Zimmermann to the German ambassador to Mexico. In it, Zimmermann said that Germany intended “to begin... unrestricted submarine warfare” and that the ambassador should “make Mexico a proposal of alliance on the following basis: make war together, make peace together, generous financial support and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona.” Zimmermann ended by instructing that the Mexican president be made aware “that the ruthless employment of our submarines now offers the prospect of compelling England in a few months to make peace.” Later that month, German U-boats attacked and sank three American merchant ships.

On April 2, Wilson called Congress in to an “extraordinary session,” to request a declaration of war against Germany. He assured members of Congress that “the world must be safe for democracy” and that “[the United States has] no quarrel with the German people...” but only with its “little groups of ambitious men who... use their fellowmen as pawns and tools.”

He continued:

It is a fearful thing to lead this great peaceful people into war, into the most terrible and disastrous of all wars, civilization itself seeming to be in the balance. But the right is more precious than peace, and we shall fight for the things which we have always carried nearest our hearts—for democracy, for the right of those who submit to authority to have a voice in their own governments, for the rights and liberties of small nations, for a universal dominion of right by such a concert of free peoples as shall bring peace and safety to all nations and make the world itself at last free.

Four days later the United States was officially at war, though it was still far from ready to take on the fight. (Read President Wilson’s proclamation urging all Americans to put their country first here.)

View Images “The Yanks Are Coming” George M. Cohan wrote the song “Over There” in April 1917 to cheer on American servicemen, like these two sailing from New York City Harbor to Europe to join the fight. Photograph by Culver Pictures/Aurimages

Battle Preparations

On June 5, 1917, the first day of the draft, millions of American men registered for service at their local draft boards as military bands played and crowds cheered. Americans moved quickly on the home front to support the war effort. The American propaganda machine preached patriotism, a wariness for spies “in our midst,” and food conservation. “Food Will Win the War,” preached the campaign spearheaded by Food Administration director Herbert Hoover and fueled by a phalanx of top American illustrators. By planting “war gardens” and giving up meat and wheat, American families could help feed their troops overseas and the starving masses of Europe.

A few months later the combined Army and National Guard forces stood at just over 400,000 men—almost exactly the number the French had lost just in the fighting at Verdun—and most of the officers were inexperienced. Only Pershing had commanded a large force, and he was soon appointed commander-in-chief of the American Expeditionary Forces (AEF). But Pershing, then 56, and most of the Army’s top brass were woefully ignorant of the kind of modern warfare being conducted in Europe and clung to an outdated tactic from the Army Infantry Journal: “In battle, it is the most resisting soul that triumphs.” They believed that small mobile units armed with rifles could somehow win the day against tanks, poison gas, airplanes, and machine guns. They also harbored the strange expectation that their untrained soldiers would not have to fight a major offensive until sometime in 1919.

In June 1917 Pershing landed in France with the First Division—the “Big Red One.” On July 4 one of its battalions paraded through the streets of Paris, arriving with fanfare at the tomb of an old ally from the War for Independence. There, Col. Charles Stanton announced on behalf of the AEF, “Lafayette, we are here!” Under orders from Wilson, the AEF troops were not to be merged with the Allies but to remain “a separate and distinct component... the identity of which must be preserved.” They were, however, to be trained in tactics by the French, who set a rigorous pace. According to one AEF officer, George Marshall (who would rise to great heights in World War II), Pershing “did not approve of the French methods of instruction.”

It would take the loss of thousands of men before American commanders changed their strategy. “It was the grim common sense of the ‘doughboy’ and not our obsolete and impossible tactics that won us ground,” Hervey Allen, a junior infantry officer serving on the western front later wrote. Before the year was out, some two million of these brave American soldiers would be fighting in the United States’ first major international war—to its bloody end.