The Zimmerman note and unrestricted submarine warfare by Germany
Explanation:
The Zimmermann Telegram was a note from German official to offer military alliance between Mexico and Germany. President Wilson understood the diplomatic relation and broke off the relation and decided to enter the WWI.
Germany was repeatedly attacking U.S. by submarine warfare in the Atlantic that killed ore than 120 U.S. citizens on May 7, 1915 that provoked outrage in the U.S. to participate in WWI.
Hence, the two reason are The Zimmerman note and unrestricted submarine warfare by Germany.
