Answer:
German submarine attacks on American ships
Explanation:
When the WWI began, the U.S. decided to maintain its isolationist policy, however, the continuous use of submarine warfare by Germany in the North Atlantic and the Mediterranean that put American ships and citizens at risk and caused the sinking of several ships caused tension to increase between both nations and brought the U.S. closer into the war.
On January 31, 1917, when Germany formally announced its unrestricted submarine warfare policy, the U.S. finally determined to end diplomatic relations with Germany and entered into World War I on the side of the Allies in April.