The Zimmermann Telegram was simply a secret diplomatic communication that was issued in 1917 from the German Foreign Office.
The Zimmermann Telegram was sent in order to propose a military alliance which will be between Mexico and Germany once there's entry of the United States into World War I against the Germans.
The message from Zimmerman to the German ambassador in Mexico was then intercepted by the British code breakers. The message was then send by the British to the United States. This led to the declaration of war against the Germans and accelerated World War I.
