What event initiated the U.S. involvement in World War I?

The event that most directly initiated U.S. involvement in World War I was the sinking of the Lusitania in 1915. This British ocean liner was torpedoed by a German U-boat, resulting in the deaths of 1,198 passengers, including 128 Americans. The incident created widespread outrage in the United States and contributed to a shift in public opinion against Germany.

Although the Lusitania was not the sole reason for the U.S. entering the war, it highlighted the dangers of unrestricted submarine warfare and underscored the potential threats to American lives and trade. This event was pivotal in turning the American public's sentiment towards supporting U.S. involvement in the conflict.

In contrast, the other events listed are significant but occurred later or were not direct triggers for U.S. involvement. The attack on Pearl Harbor happened during World War II and triggered American entry into that conflict. The signing of the Treaty of Versailles followed the end of World War I and marked the formal conclusion of the war, while the Zimmerman Telegram, which proposed a military alliance between Germany and Mexico against the U.S., further inflamed tensions but came after the public's reaction to the Lusitania tragedy had already set the stage for increased American involvement.