What was the main reason the United States entered World War I?

Why This Matters

World War I began in Europe in 1914, but the United States tried to stay neutral for the first three years of the conflict. President Woodrow Wilson won re-election in 1916 partly on the slogan "He kept us out of war." However, several events forced the United States to finally join the fighting in April 1917, changing the course of the war and America's role in the world forever.

One major reason was that Germany was sinking American civilian ships with submarines. German U-boats attacked merchant and passenger ships crossing the Atlantic Ocean, killing American citizens. The most famous incident was the sinking of the Lusitania in 1915, which killed nearly 1,200 people, including 128 Americans. Another critical reason was the Zimmermann Telegram, a secret message in which Germany tried to convince Mexico to declare war on the United States in exchange for help reclaiming territory in Texas, New Mexico, and Arizona.

The United States also wanted to support the Allied Powers, Britain, France, and Russia, who shared democratic values. When the U.S. entered the war, it sent over two million soldiers to Europe, and their arrival helped turn the tide in favor of the Allies. The war ended in November 1918. America's entry into World War I marked the beginning of its role as a major global power.

Key Facts

• The U.S. entered World War I in April 1917
• Germany's submarine warfare sank American civilian ships, including the Lusitania
• The Zimmermann Telegram revealed Germany's attempt to ally with Mexico against the U.S.
• The U.S. wanted to support the Allied Powers (Britain, France, Russia)
• Over 116,000 American soldiers died in World War I

Common Mistakes

• Confusing the reasons for entering WWI with WWII, Pearl Harbor was WWII, not WWI
• Saying the U.S. entered because of the assassination of Archduke Franz Ferdinand, that started the war in Europe, but the U.S. did not join for three more years
• Thinking the U.S. was in the war from the very beginning, the country stayed neutral until 1917

Study Tip

Remember three key reasons: ships, Mexico, and allies. Germany sank American ships, Germany tried to get Mexico to attack the U.S., and the U.S. wanted to help its allies. Pick whichever one is easiest for you to explain and use that as your answer.