In 1917, Germany, determined to win its war of attrition against the Allies, announced the resumption of unrestricted warfare in war-zone waters. Three days later, the United States broke diplomatic relations with Germany, and just hours after that the American liner Housatonic was sunk by a German U-boat.
It shows that Germany was in a really bad strategic position since it could be attacked from all sides, which did happen in the end. When the Allies managed to liberate Europe, they focused on attacking Germany from all sides and eventually they completely destroyed them, especially the Soviet army which managed to get into Berlin first.
