Zimmermann's message stated that in the event of war with the United States, Mexico should be asked to enter the conflict as a German ally. In return, Germany promised to restore to Mexico the territories it had lost to America, in the southwest US.
Seeing this, Page immediately sent a copy to President Woodrow Wilson who allowed the State Department to publish the note. The press initially treated the telegram as a fake, but Arthur Zimmermann himself confirmed its authenticity. Once Zimmermann confirmed his telegram, the outcry from the American public, combined with unhappiness over American ships being sunk and suspected German sabotage, was enough for Congress to declare war.
Unfortunately this event is barely remembered. Most are taught that the sinking of the Lusitania was the reason for US entry into WWI. However the Lusitania was sunk May 7, 1915, almost a full two years before the US declared war. The Zimmermann Note is the key event that led to the US joining the war on April 6, 1917.