What event triggered the start of World War I for the United States?

The sinking of the Lusitania and the subsequent submarine warfare were pivotal events that led the United States to enter World War I. Although the assassination of Archduke Ferdinand ignited the conflict in Europe, it was the direct impact on American interests and lives that shifted public opinion towards intervention. The Lusitania, a British passenger liner, was sunk by a German submarine in 1915, resulting in the deaths of 128 Americans among others. This event highlighted the dangers of unrestricted submarine warfare and raised strong anti-German sentiment across the United States.

Following this tragedy, Germany's continued aggressive submarine campaign, which threatened American shipping and lives, exacerbated tensions. The U.S. government was faced with considerable pressure from both the public and political leaders to respond to these provocations.

In contrast, the attack on Pearl Harbor is associated with the entry of the United States into World War II, while the signing of the Treaty of Versailles marked the end of World War I, not its beginning. The Lusitania incident and subsequent aggressive actions by Germany were critical factors that led to the U.S. declaring war on Germany in 1917 and ultimately joining the Allies in World War I.