The United States’ involvement in World War I was driven by a combination of factors, including political, economic, and social pressures. Here are the key causes:

1. Unrestricted Submarine Warfare

• German U-Boat Attacks: Germany’s use of unrestricted submarine warfare, where they attacked military and civilian ships without warning, was a significant factor. The sinking of ships such as the RMS Lusitania in 1915, which resulted in the deaths of 128 Americans, turned public opinion against Germany.
• Resumption of Attacks: Despite promising to restrict submarine warfare after the Lusitania incident, Germany resumed unrestricted attacks in 1917, sinking several American ships. This direct threat to American lives and property was a critical factor in pushing the U.S. towards war.

2. Zimmermann Telegram

• Secret Communication: In January 1917, British intelligence intercepted and decoded a secret telegram from German Foreign Minister Arthur Zimmermann to the German ambassador in Mexico.
• Proposal to Mexico: The telegram proposed a military alliance between Germany and Mexico if the U.S. entered the war against Germany. In return, Mexico would receive support to reclaim territories it had lost to the U.S., including Texas, New Mexico, and Arizona.
• Public Outrage: The revelation of the Zimmermann Telegram in March 1917 caused a public outcry in the United States and fueled anti-German sentiment.

3. Economic Interests

• Trade and Loans: The U.S. had strong economic ties with the Allies, particularly Britain and France. American banks and businesses had extended substantial loans and credits to the Allies, which they risked losing if the Allies were defeated.
• Trade Disruptions: German submarine warfare disrupted trade routes, affecting American economic interests. Protecting these interests became a priority.

4. Ideological and Political Factors

• Democracy and Autocracy: President Woodrow Wilson framed the war as a struggle between democracy and autocracy. He believed that American involvement would help make the world “safe for democracy.”
• Wilson’s Idealism: Wilson’s vision of a new world order, where international disputes would be settled peacefully through institutions like the League of Nations, played a role in his decision to enter the war.

5. Allied Propaganda

• Influence on Public Opinion: The Allies effectively used propaganda to sway American public opinion. Stories of German atrocities in Belgium and elsewhere were highlighted to generate support for the Allies.
• Cultural Ties: The U.S. shared cultural and historical ties with Britain and France, which influenced public sentiment and made the American populace more sympathetic to the Allied cause.

6. Domestic Politics

• National Unity: There was a growing sense that national unity and action were necessary. The U.S. had been divided on the issue of war, but events like the Lusitania sinking and the Zimmermann Telegram helped unify public opinion towards supporting the Allies.
• Wilson’s Leadership: President Wilson’s leadership and his eventual conclusion that neutrality was no longer tenable were crucial. His appeal to Congress in April 1917 for a declaration of war emphasized the need to defend American rights and principles.

Summary

The U.S. involvement in World War I was the result of a combination of provocations and interests. Unrestricted submarine warfare by Germany and the Zimmermann Telegram were direct triggers, while economic interests, ideological commitments, and political pressures provided broader context and support for the decision. On April 6, 1917, the U.S. Congress declared war on Germany, marking the official entry of the United States into World War I.