(A) ** German declared unlimited submarine warfare and the Zimmerman Telegram
(B) Mexico invaded the USA
(C) Japan attacked Pearl Harbor
(D) Railroads were being attacked using trench warfare
EXPLANATIONS BELOW
Concept note-1: -The United States later declared war on Austria-Hungary on December 7, 1917. Germany’s resumption of submarine attacks on passenger and merchant ships in 1917 was the primary motivation behind Wilson’s decision to lead the United States into World War I.
Concept note-2: -The telegram was considered perhaps Britain’s greatest intelligence coup of World War I and, coupled with American outrage over Germany’s resumption of unrestricted submarine warfare, was the tipping point persuading the U.S. to join the war.