In 1915, a German U-boat sank the British passenger liner Lusitania, killing 1,198 people, including 128 Americans. This incident outraged Americans and increased support for intervention in the war.
2. The Zimmerman Telegram:
In 1917, the British intercepted a telegram from the German Foreign Secretary, Arthur Zimmermann, to the German ambassador in Mexico. The telegram proposed an alliance between Germany and Mexico in the event of a war between the United States and Germany. Mexico would be rewarded with the return of territories lost to the United States in the Mexican-American War. The telegram was published in the United States and further inflamed public opinion against Germany.
3. Unrestricted Submarine Warfare:
In February 1917, Germany announced that it would resume unrestricted submarine warfare, meaning that it would sink any ship, including neutral ships, in the war zone around Britain. This announcement was a direct challenge to the United States, which had warned Germany that it would consider unrestricted submarine warfare an act of war.
These three events were the primary factors that motivated the United States to enter World War I.