In 1917, as World War I dragged on in Europe, a neutralist President Wilson and a mostly apathetic American public wanted little to do with the European conflict. In fact, Wilson had just won reelection under the slogan, “He kept us out of war.” However…

…one supremely significant event early in that year would change the attitude of the entire country toward the war in general and toward Germany in particular. That event was the publication of what came to be known as the Zimmermann Telegram, so named because its author was Arthur Zimmermann, imperial Germany’s foreign minister. In it, Zimmermann secretly proposed to Mexico, then hostile to the United States, an alliance with Germany in which the Germans would provide Mexico with ample supplies that the Mexicans would be free to use to reconquer Texas, New Mexico, and Arizona. He further suggested that the Mexican president invite Japan, nominally an Allied nation but of great strategic concern to the United States, to join the German-Mexican pact. Naturally, when the German attempt to bring the war to the territory of a neutral United States became known (and Zimmermann inexplicably acknowledged authorship), the American view of Germany was so altered that within five weeks that one message had accomplished what even the earlier German declaration of unrestricted submarine warfare had not: the United States declared war.

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Arthur Zimmermann

Inside Germany there was a thorough investigation as to how the top secret, coded telegram came into the possession of the United States government.  The Germans concluded that their codes had not been broken and attributed the compromise to treason. In fact, they could not have been more wrong, because the truth was that the revelation of the Zimmermann telegram was the greatest cryptologic triumph of the First World War.

On the first day of the war, the British cut Germany’s transatlantic telegraph cable, compelling the Germans to send all telegrams to the Western Hemisphere via neutral countries or via cables that actually passed through territory controlled by their enemies. At the same time, the British government accelerated the development of a cryptographic office whose purpose it would be to read enemy traffic. This organization came to be known as Room 40 because of its location in the Old Admiralty Buildings. Staffed with extremely capable people and aided by the fortuitous physical recovery by the Russians of the German naval codebooks in the Baltic Sea, it grew quickly in importance and capability. During the first two years of the war, Room 40 concentrated primarily on tactical naval traffic. However, once its successes had helped the British navy to bottle up the German fleet, it turned to the breaking of German traffic of a more strategic value.  On January 17, 1917, it was presented with its greatest opportunity of the war.

Source: NSA.gov