In 1917 Germany was determined to win the war of attrition against the allies and they announced the resumption of unrestricted warfare in war-zone waters. Three days later the United States broke diplomatic relations with Germany and hours after the United States broke the diplomatic relations with Germany American line Lusitania was sunk by a German U-Boat which is what prompted the united states into the war. I hope this helps I had this question a few days ago.
