Though Germany had previously promised President Wilson that it would cease attacking neutral carriers, on February 1, 1917 Germany departed from its policy of restraint and began unrestricted submarine warfare against all ships destined for Britain.  Several American ships were sunk in February and March of 1917.  At about this time, newspapers published an intercepted telegram from German Foreign Minister Arthur Zimmermann to the government of Mexico, proposing a German-Mexican alliance.  On April 2, President Wilson asked Congress to declare war on Germany; Congress complied four days later.

As you read, think about why the Zimmermann Note would have enraged Americans and contributed to a growing sentiment in the U.S. that the nation should enter the war on the side of the Allies.
 

We [Germany] intend to begin on the 1st of February unrestricted submarine warfare.  We shall endeavor in spite of this to keep the United States of America neutral.  In the event of this not succeeding, we make Mexico a proposal of alliance on the following basis: make war together, make peace together, generous financial support and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico and Arizona.... Inform the President [of Mexico] of the above most secretly.... Please call the [Mexican] Presidentís attention to the fact that the ruthless employment of our submarines now offers the prospect of compelling England in a few months to make peace.