A. 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.
B. The primary events that led to the United States declaration of war against Germany were the Zimmerman Telegram and Germany's announced intention to resume unrestricted submarine warfare.
C.The United States entered the war because of the Germans' decision to resume the policy of unrestricted submarine warfare, and the so-called "Zimmerman telegram," intercepted by the British, in which Germany floated the idea of an alliance with Mexico. ... They abandoned it in the face of US pressure earlier in the war.
