Answer:
- Germany's use of unlimited submarine warfare
Explanation:
In January 2017, Germany sent a note to their representative in Mexico asking him to propose a military alliance with Mexico and if the U.S. entered the war, Mexico could regain lost territory. British Intelligence intercepted this and when it was passed to America (who had helped Germany send the note), there was outrage and calls for war.
This was compounded by Germany's resumption of unlimited submarine warfare where they sank all ships going to Allied countries in the hopes that they could starve Britain into surrender. This put American ships in grave danger and this affront on neutrality had to be met by force.