On 1 Mar 1917, the text of the so-called Zimmermann Telegram, a message from the German foreign secretary, Arthur Zimmermann, to the German ambassador to Mexico proposing a Mexican-German alliance in the case of war between the US and Germany, was published on the front pages of newspapers across America.
In the telegram, intercepted and deciphered by British intelligence in January 1917, Zimmermann instructed the ambassador, Count Johann von Bernstorff, to offer significant financial aid to Mexico if it agreed to enter any future US-German conflict as a German ally. If victorious in the conflict, Germany also promised to restore to Mexico the lost territories of Texas, New Mexico and Arizona.
President Woodrow Wilson learned of the telegram’s contents on 26 Feb; the next day he proposed to Congress that the US should start arming its ships against possible German attacks. He also authorized the State Department to make public the Zimmermann Telegram. On 1 Mar, the news broke. Germany had already aroused Wilson’s ire—and that of the American public—with its policy of unrestricted submarine warfare and its continued attacks against American ships. Some of those in the US who still held out for neutrality at first claimed the telegram was a fake. This notion was dispelled two days later, when Zimmermann himself confirmed its authenticity.
Public opinion in the US now swung firmly toward American entrance into WWI. On 2 April, Wilson went before Congress to deliver a message of war. The US formally entered the conflict four days later.