The Zimmerman telegram was a communication from German Foreign Secretary Arthur Zimmerman to the Mexican government meant to persuade Mexico to form an alliance with Germany during World War I. It played a vital role in inflaming the American people against Germany and pushing President Woodrow Wilson to declare war on Germany. Shortly after the telegram became public, the United States entered the war on the side of the Allies.
In 1916, Germany briefly engaged in unrestricted submarine warfare, but halted the practice after strong protests by the United States. Preparing to resume it in early 1917, Germany sought a contingency plan in case the U.S. declared war. Zimmerman sent the telegram via American and British telegraph lines to its ambassador in Mexico City, who in turn passed the message on to the Mexican government. If Mexico agreed, Germany offered Mexico financial aid and the U.S. states of New Mexico, Arizona and Texas after the war. After considering the offer, Mexico refused, realizing it would be futile to attempt to oppose the United States in its sphere of power in the Western Hemisphere.
In the meantime, the British intercepted and decoded the telegram. They shared the information with the U.S. government, and on March 1, 1917, the story was released to the American press. The incensed American public clamored overwhelmingly for war. On April 2, President Wilson formally asked Congress for a declaration of war, and four days later it was granted.