Zimmermann Telegram

The 1917 decrypt that helped bring America into World War I

Overview

The Zimmermann Telegram, decrypted in Room 40 of British naval intelligence in January 1917, revealed Germany offering Mexico an alliance to recover Texas, Arizona, and New Mexico if America entered the war. Britain delivered the decrypt while concealing its source, and publication in the American press inflamed public opinion. Congress declared war on Germany that April. The telegram remains the textbook demonstration of one decrypted message changing the course of a superpower. British cryptanalysts Nigel de Grey and William Montgomery made the initial break in Room 40.