What major event led to U.S. involvement in World War I?

The sinking of the Lusitania was a significant catalyst for United States involvement in World War I because it highlighted the dangers of unrestricted submarine warfare, a policy employed by Germany at the time. In May 1915, the British ocean liner Lusitania was torpedoed by a German U-Boat, resulting in the deaths of nearly 1,200 passengers, including 128 Americans. This tragic event stirred public outrage in the United States and contributed to a growing sentiment against Germany. It underscored the direct threat to American lives and commerce posed by the ongoing conflict, ultimately leading President Woodrow Wilson to push for U.S. entry into the war as he sought to protect American interests and promote a safer world order.

The other options represent events that are not directly associated with the onset of U.S. involvement in World War I. For example, the assassination of Archduke Franz Ferdinand occurred in 1914 and was primarily the spark that ignited the war in Europe but did not directly involve the U.S. The bombing of Pearl Harbor happened in 1941 and was a pivotal event that brought the U.S. into World War II, while the invasion of Poland by Germany occurred in 1939, also relating to World War II