The Zimmerman was not part of a Pearl Harbor type conspiracy. Germany said it would only kick in if the U.S. entered the war against Germany. Here is what it said:
"On the first of February we intend to begin submarine warfare unrestricted. In spite of this, it is our intention to endeavour 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 general financial support, and it is understood that Mexico is to reconquer the lost territory in New Mexico, Texas, and Arizona."