The key factor in the decision of the U.S. to enter World War I was
a. A long-standing friendship with Great Britain
b. American prejudice against German immigrants
c. The political ambition of Woodrow Wilson
d. Germany's resumption of unrestricted submarine warfare against the U.S.
The key factor in the decision of the U.S. to enter World War I was "d. Germany's resumption of unrestricted submarine warfare against the U.S.," since this led to the sinking of the Lusitania, which had many US citizens on board.
