The United States was initially reluctant to enter World War I, but the sinking of several American ships by German U-boats in 1915 and 1916 began to turn public opinion against Germany. The most famous of these attacks was the sinking of the RMS Lusitania, a British passenger ship, in May 1915, which killed over 1,200 people, including 128 Americans.
2. The Zimmerman Telegram.
In January 1917, the British intercepted and decoded a telegram from German Foreign Secretary Arthur Zimmermann to the German ambassador in Mexico. The telegram proposed that Mexico ally itself with Germany and attack the United States, with the promise of regaining territory it had lost to the United States in the Mexican-American War. The publication of the telegram in the United States caused outrage and further increased support for war against Germany.
3. American economic interests.
The United States had a significant economic stake in the Allied victory in World War I. The Allies owed the United States billions of dollars in loans, and the United States feared that a German victory would make it difficult or impossible for the Allies to repay their debts. Additionally, the United States had a strong interest in maintaining the freedom of the seas, which was threatened by German submarine warfare.
4. Idealism.
Many Americans believed that it was the duty of the United States to fight for democracy and freedom in Europe. They saw World War I as a struggle between good and evil, and they believed that the United States had a moral obligation to help the Allies defeat Germany.
5. The Russian Revolution.
The Russian Revolution in 1917 led to the overthrow of the Tsarist regime and the establishment of a communist government. The United States feared that the spread of communism to Europe would pose a threat to its own security, and it saw World War I as an opportunity to weaken the communist movement.