The spark that ignited World War I came in the Balkans, where Austria-Hungary and Russia competed for control. Austria’s 1908 seizure of Ottoman provinces, including Bosnia, angered the nearby Slavic nation of Serbia and its ally, Russia. Serbian revolutionaries recruited Bosnian Slavs to resist Austrian rule. In June 1914, in the city of Sarajevo, university student Gavrilo Princip assassinated Archduke Franz Ferdinand, heir to the Austro-Hungarian throne.

Like dominos falling, the system of European alliances pushed all the powers into war. Austria-Hungary blamed Serbia for the assassination and declared war on July 28. Russia, tied by secret treaty to Serbia, mobilized against Austria-Hungary. This prompted Germany to declare war on Russia and its ally France. As a preparation for attacking France, Germany launched a brutal invasion of the neutral country of Belgium, which caused Great Britain to declare war on Germany. Within a week, most of Europe was at war, with the major Allies — Great Britain, France, and Russia — confronting the Central Powers of Germany and Austria-Hungary. Two military zones emerged. On the Western Front, Germany battled the British and French; on the Eastern Front, Germany and Austria-Hungary fought Russia. Because most of the warring nations held colonial empires, the conflict soon spread to the Middle East, Africa, and Asia.

The so-called Great War wreaked terrible devastation. New technology, some of it devised in the United States, made warfare deadlier than ever before. Every soldier carried a long-range, high-velocity rifle that could hit a target at 1,000 yards — a vast technical advancement over the 300-yard range of rifles used in the U.S. Civil War. The machine gun was even more deadly. Its American-born inventor, Hiram Maxim, had moved to Britain in the 1880s to follow a friend’s advice: “If you want to make your fortune, invent something which will allow those fool Europeans to kill each other more quickly.” New technologies helped soldiers in defensive positions; once advancing Germans ran into French fortifications, they stalled. Across a swath of Belgium and northeastern France, millions of soldiers on both sides hunkered down in fortified trenches. During 1916, repeatedly trying to break through French lines at Verdun, Germans suffered 450,000 casualties. The French fared even worse, with 550,000 dead or wounded. It was all to no avail. From 1914 to 1918, the Western Front barely moved.

At the war’s outbreak, President Wilson called on Americans to be “neutral in fact as well as in name.” If the United States remained out of the conflict, Wilson reasoned, he could influence the postwar settlement, much as Theodore Roosevelt had done after previous conflicts. Even if Wilson had wished to, it would have been nearly impossible in 1914 to unite Americans behind the Allies. Many Irish immigrants viewed Britain as an enemy — based on its continued occupation of Ireland — while millions of German Americans maintained ties to their homeland. Progressive-minded Republicans, such as Senator Robert La Follette of Wisconsin, vehemently opposed taking sides in a European fight, as did socialists, who condemned the war as a conflict among greedy capitalist empires. Two giants of American industry, Andrew Carnegie and Henry Ford, opposed the war. In December 1915, Ford sent a hundred men and women to Europe on a “peace ship” to urge an end to the war. “It would be folly,” declared the New York Sun, “for the country to sacrifice itself to … the clash of ancient hatreds which is urging the Old World to destruction.”

The Struggle to Remain Neutral The United States, wishing to trade with all the warring nations, might have remained neutral if Britain had not held commanding power at sea. In September 1914, the British imposed a naval blockade on the Central Powers to cut off vital supplies of food and military equipment. Though the Wilson administration protested this infringement of the rights of neutral carriers, commerce with the Allies more than made up for the economic loss. Trade with Britain and France grew fourfold over the next two years, to $3.2 billion in 1916; by 1917, U.S. banks had lent the Allies $2.5 billion. In contrast, American trade and loans to Germany stood then at a mere $56 million. This imbalance undercut U.S. neutrality. If Germany won and Britain and France defaulted on their debts, American companies would suffer catastrophic losses.

To challenge the British navy, Germany launched a devastating new weapon, the U-boat (short for Unterseeboot, “undersea boat,” or submarine). In April 1915, Germany issued a warning that all ships flying flags of Britain or its allies were liable to destruction. A few weeks later, a U-boat torpedoed the British luxury liner Lusitania off the coast of Ireland, killing 1,198 people, including 128 Americans. The attack on the passenger ship (which was later revealed to have been carrying munitions) incensed Americans. The following year, in an agreement known as the Sussex pledge, Germany agreed not to target passenger liners or merchant ships unless an inspection showed the latter carried weapons. But the Lusitania sinking prompted Wilson to reconsider his options. After quietly trying to mediate in Europe but finding neither side interested in peace, he endorsed a $1 billion U.S. military buildup.

American public opinion still ran strongly against entering the war, a fact that shaped the election of 1916. Republicans rejected the belligerently prowar Theodore Roosevelt in favor of Supreme Court justice Charles Evans Hughes, a progressive former governor of New York. Democrats renominated Wilson, who campaigned on his domestic record and as the president who “kept us out of war.” Wilson eked out a narrow victory; winning California by a mere 4,000 votes, he secured a slim majority in the electoral college.

America Enters the War Despite Wilson’s campaign slogan, events pushed him toward war. In February 1917, Germany resumed unrestricted submarine warfare, a decision dictated by the impasse on the Western Front. In response, Wilson broke off diplomatic relations with Germany. A few weeks later, newspapers published an intercepted dispatch from German foreign secretary Arthur Zimmermann to his minister in Mexico. The Zimmermann telegram urged Mexico to join the Central Powers, promising that if the United States entered the war, Germany would help Mexico recover “the lost territory of Texas, New Mexico, and Arizona.” With Pancho Villa’s border raids still fresh in Americans’ minds, this threat jolted public opinion. Meanwhile, German U-boats attacked U.S. ships without warning, sinking three on March 18 alone.

On April 2, 1917, Wilson asked Congress for a declaration of war. He argued that Germany had trampled on American rights and imperiled U.S. trade and citizens’ lives. “We desire no conquest,” Wilson declared, “no material compensation for the sacrifices we shall freely make.” Reflecting his progressive idealism, Wilson promised that American involvement would make the world “safe for democracy.” On April 6, the United States declared war on Germany. Reflecting the nation’s divided views, the vote was far from unanimous. Six senators and fifty members of the House voted against entry, including Representative Jeannette Rankin of Montana, the first woman elected to Congress. “You can no more win a war than you can win an earthquake,” Rankin said. “I want to stand by my country, but I cannot vote for war.”