"On the first of February, we intend to begin submarine warfare unrestricted. In spite of this it is our intention to keep neutral the United States of America. "If this attempt is not successful we propose an alliance on the following
basis with Mexico: that we shall make war together and together make peace. We shall give financial support, and it is understood that Mexico is to reconquer the lost territory in New Mexico, Texas and Arizona. The details are left for your settlement."
-Arthur Zimmermann, German Foreign Minister, January 19, 1917.
Which of the following does this excerpt support as the primary cause of the U.S. declaration of war in April 1917?