ZIMMERMANN TELEGRAM:
For two and a half years during the early 19th century the United States had maintained a neutral position by not yet interfering in World War I. However, their neutralness soon changed once they had discovered a secretive message, infamously known as the Zimmerman telegram. In January of 1917, German foreign secretary, Arthur Zimmermann had sent a confidential message to the German ambassador in order for it to be passed to the Mexicans. The telegram had attempted to convince the Mexicans to form a German-Mexican alliance to attack the U.S, with the bribe of regaining the territory they had lost during the Mexican-American war (1846). Yearning to know what the telegram contained, British intelligence had intercepted and passed it onto British cryptanalyst, Nigel de Grey, who decoded the message and discovered that it was a secretive proposal sent from the Germans in an attempt to convince the Mexicans to ally against the U.S.
The British had successfully informed the Americans of the telegram without arousing suspicions from the Germans. The Americans publicized it making it widely known to all U.S citizens that war was imminent.
President Woodrow Wilson had asked congress to declare war on Germany using the Zimmerman telegram as evidence, and on April 6 1917, congress officially declared war on Germany and their allies. The Zimmerman telegram had been noted by some as a decisive element on the U.S decision to enter the war.