Answer:
German aggression
Step-by-step explanation:
During World War I, the Central Powers had submarines (called u-boats at the time) in the Atlantic Ocean. The Germans followed a policy of unrestricted submarine warfare, which meant they would sink and attack any non-German vessel. They came across the Lusitania, a ship with American passengers, and sunk it. This outraged American citizens and turned American favor towards the Allies. Another event that led to the United States joining the war was the Zimmerman Note. Great Britain intercepted a telegram from Germany that was sent to Mexico. The telegram stated that if Mexico joined the Central Powers and attacked the United States, they would be given American land.