Final answer:
The final event that convinced the US to declare war on Germany was Germany's announcement to resume unrestricted submarine warfare.
Step-by-step explanation:
The final event that convinced the US to declare war on Germany was Germany's announcement on January 31, 1917, that it would resume the use of unrestricted submarine warfare. This policy meant that German submarines would attack any ships found in the war zone, including those belonging to neutral countries like the United States. The sinking of the British ocean liner Lusitania in 1915 had already caused outrage worldwide, but Germany's decision to resume unrestricted submarine warfare was seen as a direct threat to American lives and interests, leading to the United States entering the war on April 6, 1917.
Learn more about US entry into World War I