Catalysts for U.S. Entry into World War I

In the context of World War I, the United States' decision to join the Allies in 1917 was influenced by several factors beyond just the immediate threats to its security. Events such as unrestricted submarine warfare, the Zimmerman Telegram, and the economic ties to the Allies played crucial roles. Each of these factors contributed to shifting American public opinion and policy towards entry into the war.

Which of the following events or developments was most directly responsible for mobilizing public support for U.S. participation in World War I?