Why did the United States enter World War I in 1917?

The United States entered World War I in 1917 primarily due to Germany’s resumption of unrestricted submarine warfare and the discovery of the Zimmermann Telegram.12

By early 1917, Germany’s military command, believing that U-boats could force Britain to surrender by cutting off supplies, resumed unrestricted attacks on merchant vessels, including American ships. This policy, which involved sinking ships without warning, directly challenged neutral rights and resulted in the loss of American lives and property.12

Simultaneously, the Zimmermann Telegram—a secret communication from German foreign secretary Arthur Zimmermann to the German minister in Mexico—proposed a military alliance against the United States. The telegram promised Mexico the return of its "lost provinces" of Texas, Arizona, and New Mexico if it joined Germany in the war.13

After British intelligence intercepted and leaked the telegram, its publication in March 1917 outraged the American public. Following these provocations, President Woodrow Wilson addressed Congress, declaring that the world must be made "safe for democracy," and the United States formally declared war on Germany on April 6, 1917.321