Answer:
Submarine warfare
Explanation:
During WWI, the U.S. had 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, caused tension to increase between both nations and brought the U.S. closer into the war. And when Germany formally announced its unrestricted submarine warfare policy on January 31, 1917, the U.S. finally determined to end diplomatic relations with Germany three days later.
