What major event triggered the United States' involvement in World War I?

The sinking of the Lusitania was a significant event that contributed to the growing anti-German sentiment in the United States and played a crucial role in pushing the nation toward involvement in World War I. On May 7, 1915, a British passenger ship, the Lusitania, was torpedoed by a German U-boat, resulting in the deaths of 1,198 passengers, including 128 Americans. The incident garnered extensive media coverage and outrage in the United States, highlighting the risks of unrestricted submarine warfare employed by Germany.

While the Lusitania incident was a catalyst for public opinion against Germany, it was not the sole reason the U.S. entered the war. Other factors contributed, including economic ties to the Allies and continued German provocations. However, the sinking of the Lusitania helped shift public sentiment by emphasizing the dangers posed to American lives and commerce.

In contrast, the Zimmermann Telegram, while significant and a direct provocation, came later and served as a more immediate trigger for U.S. involvement. It revealed Germany's proposal to Mexico for a military alliance against the United States. The assassination of Franz Ferdinand, while the immediate cause of the war in Europe, did not directly involve the United States at the time.