What event prompted the U.S. to enter World War I?

The U.S. entry into World War I was primarily motivated by the sinking of the Lusitania and the broader context of unrestricted submarine warfare. The Lusitania, a British passenger ship, was sunk by a German U-boat in 1915, resulting in the deaths of 128 Americans. This incident, along with an ongoing policy by Germany of targeting neutral shipping vessels, heightened American outrage and shifted public opinion against Germany.

Unrestricted submarine warfare, initiated by Germany in early 1917, further escalated tensions. It involved the declaration that German submarines would attack any ship, including civilian and neutral vessels, without warning in designated war zones. This direct violation of established maritime laws and the loss of American lives galvanized support for intervention in the conflict.

While other events, such as the invasion of Belgium or the assassination of Archduke Franz Ferdinand, were significant in the broader context of the war, they did not directly compel the United States to declare war. The combination of the Lusitania sinking and Germany's aggressive submarine tactics distinctly pushed the U.S. towards involvement, making this option the correct choice.