Answer:
Unrestricted submarine warfare
Explanation:
Germany's resumption of submarine attacks on passenger and merchant ships in 1917 became the primary motivation behind Wilson's decision to lead the United States into World War I.
The Zimmerman Telegram was a secret diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico if the United States entered World War I against Germany but was not the cause of the U.S. joining the war.
