What event triggered the United States' entry into World War I?

The sinking of the Lusitania was a key event that influenced public opinion in the United States and served as a significant catalyst for its entry into World War I. The Lusitania was a British ocean liner that was sunk by a German U-boat in May 1915, resulting in the deaths of 1,198 passengers, including 128 Americans. This incident incited outrage among the American public and government officials, shifting sentiment towards supporting the Allies and against Germany.

The attack highlighted the dangers of unrestricted submarine warfare, a tactic employed by Germany that threatened neutral shipping and American lives. Although the United States did not enter the war immediately after the Lusitania was sunk, the event contributed to a growing movement within the country to involve itself in the conflict. It was later events, particularly the resumption of unrestricted submarine warfare by Germany and the Zimmerman Telegram, that ultimately prompted the U.S. to declare war in April 1917.

Other events listed, such as the assassination of Archduke Franz Ferdinand, led to the outbreak of the war in Europe, but they did not directly involve the United States. The invasion of Poland occurred much later, during World War II, and the attack on Pearl Harbor was a pivotal event for U.S.