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. The United States entered the war because of the Germans’ decision to resume the policy of unrestricted submarine warfare, and Zimmerman, intercepted by the British , in which Germany floated the idea of an alliance with Mexico.
