Of which the first line was:

Berlin, January 19, 1917

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.
The only stipulation in the letter was that if the U.S. declared war on Germany, Mexico was to also declare war on the U.S. It wasn't an attempt to secretly strike at the U.S., it was setting up a countermove for a possible U.S. strike.

EDIT:

The Germans weren't morons. They knew the possibility that the U.S. would intervene would significantly decrease the odds of their winning. Germany would have struck peace with the U.S. if they ever thought it was possible.