Question

which was the major cause for the entry of the U.S into world war 1?

Answer 1

The United States entered the war because of the Germans' decision to resume the policy of unrestricted submarine warfare.

Answer 2

The United States entered world war1 because of the Germans' decision to resume the policy of unrestricted submarine warfare, and  what they called the 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.