America Enters World War I: Why the United States Finally Joined the Fight

When World War I erupted in Europe in 1914, President Woodrow Wilson declared that the United States would remain neutral. Millions of Americans supported that position. The conflict seemed distant, and many believed the country had little reason to become involved in Europe's devastating war.

Less than three years later, that position had changed. On April 6, 1917, the United States officially entered World War I on the side of the Allied powers.

What pushed America from neutrality to war?

America Tries to Stay Neutral

Wilson initially urged Americans to remain neutral not only in action but also in thought.

Maintaining neutrality became increasingly difficult. American businesses traded heavily with Britain and France, while Britain's naval blockade limited commerce with Germany.

The war also reached Americans at sea. German submarines, known as U-boats, targeted ships traveling around Britain in an effort to cut Allied supply lines.

In May 1915, a German U-boat sank the British passenger liner Lusitania. Nearly 1,200 people died, including more than 100 Americans.

The sinking increased anti-German sentiment in the United States, although America did not immediately enter the war.

Germany Resumes Unrestricted Submarine Warfare

By early 1917, Germany was struggling to break the stalemate in Europe.

German leaders resumed unrestricted submarine warfare, meaning U-boats could attack ships in designated waters without warning, including vessels from neutral countries.

Germany knew this policy could bring the United States into the conflict. Its leaders gambled that they could weaken Britain before American forces could make a significant difference.

American ships soon became targets, making continued neutrality increasingly difficult.

The Zimmermann Telegram

Another development further changed American opinion.

British intelligence intercepted a secret message from German Foreign Secretary Arthur Zimmermann to the German government in Mexico.

The Zimmermann Telegram proposed an alliance if the United States entered the war against Germany. In return, Germany suggested that Mexico could recover territory it had lost to the United States, including Texas, New Mexico, and Arizona.

When the message became public in March 1917, it caused widespread outrage.

Wilson Asks Congress for War

On April 2, 1917, Wilson appeared before Congress and requested a declaration of war against Germany.

He argued that German submarine warfare threatened American lives and rights. Wilson also framed American involvement in broader terms, declaring that the world must be made "safe for democracy."

Congress approved the declaration, and the United States formally entered the war on April 6.

America's Entry Changes the War

The United States was not immediately prepared to send a massive army overseas. Mobilizing millions of soldiers, producing equipment, and transporting forces across the Atlantic took time.

Eventually, American troops arrived in Europe in large numbers. Their manpower and resources strengthened exhausted Allied forces during the war's final stages.

Germany agreed to an armistice on November 11, 1918.

America's entry into World War I marked another major change in its relationship with the wider world. The country had tried to remain outside Europe's conflict, but submarine attacks, diplomatic threats, and growing international connections made neutrality increasingly difficult to maintain.

By 1917, the United States could no longer remain on the sidelines.