When World War I broke out across Europe in 1914, President Woodrow Wilson proclaimed the United States would remain neutral, and many Americans supported this policy of nonintervention. However, public opinion about neutrality started to change after the sinking of the British ocean liner Lusitania by a German U-boat in 1915; almost 2,000 people perished, including 128 Americans. Along with news of the Zimmermann telegram threatening an alliance between Germany and Mexico against America, Wilson asked Congress for a declaration of war against Germany. The United States officially entered the conflict on April 6, 1917.

World War I Begins

On June 28, 1914, Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire, and his wife Sophie were assassinated by a Bosnian Serb nationalist in Sarajevo, the capital of the Austro-Hungarian province of Bosnia and Herzegovina.

One month later, on July 28, Austria-Hungary declared war on Serbia. Within a week, Russia, France, Belgium, Great Britain and Serbia had sided against Austria-Hungary and Germany, and the Great War, as it was originally called, was underway.

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Central Powers

Germany and Austria-Hungary later teamed with the Ottoman Empire and Bulgaria and were referred to collectively as the Central Powers. Russia, France and Great Britain, the major Allied Powers, eventually were joined by Italy, Japan and Portugal, among other nations.

On August 4, as World War I erupted across Europe, President Woodrow Wilson proclaimed America’s neutrality, stating the nation “must be neutral in fact as well as in name during these days that are to try men’s souls.”

With no vital interests at stake, many Americans supported this position. Additionally, America was home to a number of immigrants from countries at war with each other and Wilson wanted to avoid this becoming a divisive issue.

American companies, however, continue to ship food, raw materials and munitions to both the Allies and Central Powers, although trade between the Central Powers and the United States was severely curtailed by Britain’s naval blockade of Germany. U.S. banks also provided the warring nations with loans, the bulk of which went to the Allies.

Lusitania Sinks

On May 7, 1915, a German submarine sank the British ocean liner Lusitania, resulting in the deaths of nearly 1,200 people, including 128 Americans. The incident strained diplomatic relations between Washington and Berlin and helped turn public opinion against Germany.

President Wilson demanded that the Germans stop unannounced submarine warfare; however, he didn’t believe the United States should take military action against Germany.

Some Americans disagreed with this nonintervention policy, including former president Theodore Roosevelt, who criticized Wilson and advocated going to war. Roosevelt promoted the Preparedness Movement, whose aim was to persuade the nation it must get ready for war.

'America First'

In 1916, as American troops were deployed to Mexico to hunt down Mexican rebel leader Pancho Villa following his raid on Columbus, New Mexico, concerns about the readiness of the U.S. military grew. In response, Wilson signed the National Defense Act in June of that year, expanding the Army and the National Guard, and in August, the president signed legislation designed to significantly strengthen the Navy.

After campaigning on the slogans “He Kept Us Out of War” and “America First,” Wilson was elected to a second term in the White House in November 1916.

Meanwhile, some Americans joined the fighting in Europe their own. Starting in the early months of the war, a group of U.S. citizens enlisted in the French Foreign Legion. (Among them was the poet Alan Seeger, whose poem “I Have a Rendezvous with Death” later was a favorite of President John F. Kennedy. Seeger was killed in the war in 1916.) Other Americans volunteered with the Lafayette Escadrille, a unit of the French Air Service, or drove ambulances for the American Field Service.

Submarine Warfare Resumes

In March 1916, a German U-boat torpedoed a French passenger ship, Sussex, killing dozens of people, including several Americans. Afterward, the United States threatened to cut diplomatic ties with Germany.

In response, the Germans issued the Sussex pledge, promising to stop attacking merchant and passenger ships without warning. However, on January 31, 1917, the Germans reversed course, announcing they would resume unrestricted submarine warfare, reasoning it would help them win the war before America, which was relatively unprepared for battle, could join the fighting on behalf of the Allies.

In response, America severed diplomatic ties with Germany on February 3. During February and March, German U-boats sank a series of U.S. merchant ships, resulting in multiple casualties.

Zimmermann Telegram

Meanwhile, in January 1917, the British intercepted and deciphered an encrypted message from German Foreign Minister Arthur Zimmermann to the German minister in Mexico, Heinrich von Eckhart.

The so-called Zimmermann telegram proposed an alliance between Germany and Mexico—America’s southern neighbor—if America joined the war on the side of the Allies.

As part of the arrangement, the Germans would support the Mexicans in regaining the territory they’d lost in the Mexican-American War—Texas, New Mexico and Arizona. Additionally, Germany wanted Mexico to help convince Japan to come over to its side in the conflict.

The British gave President Wilson the Zimmermann telegram on February 24, and on March 1 the American press reported on its existence. The American public was outraged by the news of the Zimmermann telegram and it, along with Germany’s resumption of submarine attacks, helped lead to the United States joining the war.

America Declares War on Germany

On April 2, 1917, Wilson went before a special joint session of Congress and asked for a declaration of war against Germany, stating: “The world must be made safe for democracy.”

On April 4, the Senate voted 82 to 6 to declare war. Two days later, on April 6, the House of Representatives voted 373 to 50 in favor of adopting a war resolution against Germany.

Among the dissenters was Rep. Jeannette Rankin of Montana, the first woman in Congress. It was only the fourth time Congress had declared war; the others were the War of 1812, the Mexican-American War and the Spanish-American War of 1898.

In early 1917, the U.S. Army had just 133,000 members. That May, Congress passed the Selective Service Act, which reinstated the draft for the first time since the Civil War and led to some 2.8 million men being inducted into the U.S. military by the end of the Great War. Around 2 million more Americans voluntarily served in the armed forces during the conflict.

The first U.S. infantry troops arrived on the European continent in June 1917; in October, the first American soldiers entered combat in France. That December, America declared war against Austria-Hungary (America never was formally at war with the Ottoman Empire or Bulgaria).

When the war concluded in November 1918, with a victory for the Allies, more than 2 million U.S. troops had served at the Western Front in Europe, and more than 50,000 of them died.

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U.S. Entry into World War I, 1917. U.S. Department of State, Office of the Historian.
Why did the US enter World War I? The University of Rochester Newscenter.
U.S. Participation in the Great War (World War I). Library of Congress.