17 January 1917 The intercepted telegram sent by Alfred Zimmermann in the German Foreign Office to the German embassies in Washington, D.C., and Mexico City is delivered to the British Naval cryptographic office, Room 40.
The message itself is in two parts; the first part are instructions to Count von Bernstorff, the German Ambassador to the United States on how to forward the message in the second part to the German Envoy in Mexico, Heinrich von Eckhardt and also informs von Bernstorff of the German's intention to resume unrestricted submarine warfare. The second part is the Zimmermann Telegram everyone knows.
The British codebreakers in Room 40, William Montgomery and Nigel de Grey, immediately begin to decrypt the message, although because they have not completely solved the new German code, No. 0075, the decryption remains incomplete for several weeks. Code 0075, introduced in mid-1916, is a two-part code of 10,000 code groups. Despite this the British learn that its message outlines plans for an alliance between Germany and Mexico against the United States. According to the scheme, Germany would provide tactical support while Mexico would benefit by expanding into the American Southwest, retrieving territories that had once been part of Mexico.
