Which event led to the U.S. entry into World War I?

The sinking of the Lusitania significantly impacted American sentiment and propelled the United States closer to entering World War I. This British ocean liner was torpedoed by a German U-boat in May 1915, resulting in the deaths of nearly 1,200 passengers, including 128 Americans. This tragic event stirred outrage across the United States and raised questions about Germany's unrestricted submarine warfare policy, which threatened neutral shipping and civilian lives.

Although the Lusitania sinking did not lead to an immediate declaration of war, it fueled anti-German sentiment and highlighted the dangers posed by the ongoing conflict in Europe. This event, combined with other factors such as the Zimmermann Telegram and growing economic ties with the Allies, ultimately contributed to the U.S. decision to join the war in April 1917.

The other events listed do not relate to the U.S. entry into World War I. The bombing of Pearl Harbor was a pivotal moment leading to U.S. involvement in World War II, the invasion of Poland marked the beginning of World War II in 1939, and the signing of the Treaty of Versailles formally ended World War I but occurred after the U.S. had already joined the fight.