Most historians agree that American involvement in World War I was unavoidable by early 1917. U.S. entry into war was probably quickened by a letter written by the German foreign secretary, Arthur Zimmermann.
On January 16, 1917, British code breakers intercepted an encrypted message from Zimmermann and intended for the German ambassador to Mexico. The letter gave the ambassador a set of instructions: if the neutral United States entered the war on the side of the Allies, he should approach Mexico’s president with an offer to create a secret wartime alliance. The Germans would provide military and financial support for a Mexican attack on the United States. In return, Mexico would be free to annex “lost territory in Texas, New Mexico and Arizona.” In addition, the ambassador should use the Mexicans as a go-between to encourage the Japanese Empire to join the German cause.
The British cryptographic office decoded the Zimmermann Telegram and handed it over to the United States. Its contents were splashed on the front pages of newspapers nationwide. Diplomatic relations between Germany and the United States had already been cut off when Germany attacked U.S. vessels in the Atlantic Ocean. While many Americans remained committed to isolationism, the secret Zimmerman message was fresh evidence of German aggression. Together with the submarine attacks, it finally convinced the U.S. government to join the war. On April 2, 1917, President Wilson abandoned his policy of neutrality and asked Congress to declare war against Germany and the Central Powers. The United States joined the Allies four days later.
Source: What Was the Zimmermann Telegram?
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