What event led the United States to enter World War I?

The interception of the Zimmermann Telegram was a pivotal event that significantly swayed public opinion and the U.S. government's stance regarding involvement in World War I. In this telegram, sent by German Foreign Minister Arthur Zimmermann to Mexico, Germany promised to support Mexico in reclaiming territories such as Texas, New Mexico, and Arizona in exchange for Mexico joining the war against the United States.

When the British intelligence intercepted and then revealed the contents of the telegram in early 1917, it stirred outrage among the American public and highlighted the threat posed by Germany. This, along with other factors such as unrestricted submarine warfare—where German U-boats targeted not only military vessels but also civilian ships, leading to the sinking of the Lusitania—encouraged the U.S. to reconsider its neutral stance.

Although the sinking of the Lusitania was an important factor, as it resulted in the loss of American lives and heightened anti-German sentiments, it was the specific and direct provocation of the Zimmermann Telegram that ultimately galvanized the U.S. into formally declaring war on Germany on April 6, 1917. Thus, this event stands out as the crucial turning point that led to American involvement in the conflict.