In the spring of 1917, German Foreign Minister Alfred Zimmerman sent a coded telegram to the German envoy in Mexico. It was intercepted and decoded by the British, who turned over the text to American Ambassador Walter Hines Page in London.
Zimmerman proposed a Mexican-Germany alliance, by taking advantage of the American government's difficulties with Pancho Villa in exchange for helping Mexico regain some of its former territories - Arizona, Texas and New Mexico. On 2 April 1917, Pres. Wilson asked Congress to declare war against Germany and the Central Powers. Four days later, war was declared.
Although the note from Zimmerman appears to be a contingency plan in case the United States did enter the war, it's result was so inflammatory that America did declare war. The translated text of the Zimmerman telegram is as follows:
"On the first of February we intend to begin submarine warfare unrestricted. In spite of this, it is our intention to endeavor 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. The details are left to you for settlement ..."
"You are instructed to inform the President of Mexico of the above in the greatest confidence as soon as it is certain that there will be an outbreak of war with the United States and suggest that the President of Mexico, on his own initiative, should communicate with Japan suggesting adherence at once to this plan; at the same time, offer to mediate between Germany and Japan."
"Please call to the attention of the President of Mexico that the employment of ruthless submarine warfare now promises to compel England to make peace in a few months."