Excerpts from Books and Wikipedia
"The telegram was a gamble born of Germany's desperation to keep the United States out of the war. Berlin knew that Washington's patience would be sorely tried by Germany's sinking of neutral (including US) shipping after its declaration of unrestricted submarine warfare (USW). The telegram tried that patience to the breaking point. In it, the newly appointed Zimmerman declare that should USW draw the United States into war, then Germany would encourage Mexico to declare war on its northern neighbor. Mexico's bait was Germany's promise to restore the 'lost territories' conquered or annexed by the United States in the 19 century: the states of Texas, New Mexico, and Arizona. . . . An outraged President Woodrow Wilson's declaration of war against Germany followed in April. Historians believe that the duplicitous malice of the telegram was the decisive factor in tipping a reluctant Wilson into war." [Furtado: 1001 Days]