Which factor was a primary catalyst for the United States to enter World War I in 1917?

• A. Unrestricted submarine warfare by Germany
• B. The sinking of the Lusitania in 1915
• C. The Zimmermann Telegram
• D. A long-standing alliance with Britain

Answer: (A)

The main idea being tested is what pushed the United States to join World War I in 1917. The decisive factor was Germany’s decision to resume unrestricted submarine warfare, meaning U‑boats could sink Allied and neutral ships without warning. This threatened American lives, naval crews, and trade routes, and it violated U.S. neutrality and interests on the seas. When American ships and citizens came under direct attack, the government felt compelled to act to defend national security and economic interests, making entry into the war the next logical step. While earlier events like the Lusitania’s sinking and the Zimmermann Telegram shaped public opinion and helped build support for intervention, the 1917 submarine campaign was the catalyst that directly moved the United States toward war.