Is this a multiple choice question?
Answer:Sinking of American merchant ships
Explanation:In early 1917 Berlin forced the issue. Its declared decision on 31 January 1917 to target neutral shipping in a designated war-zone became the immediate cause of the entry of the United States into the war.
Explanation:
I had to answer this question on my history exam and this is what I put.
Nationalism caused World War 1 . People put nationalism in their country above other countries which caused a lot of conflict