Germany will soon be commencing a campaign of unrestricted submarine warfare. German leaders know that there is a high likelihood of this bringing the United States of America into the war against them. They are confident however that the U-boats will defeat Britain before the Americans can raise an army to fight in Europe. And in any case, the U-boats should be able to disrupt the transport of any US troops across the Atlantic.
Nevertheless, the Germans are thinking of other ways of dealing with the American threat. Attention turns to Mexico, America’s southern neighbour, with whom the United States has a troubled relationship. Could Mexico be enlisted as an ally of Germany, thereby drawing American attentions away from Europe?
Arthur Zimmermann is Germany’s deputy foreign minister. He sends a telegram to the Heinrich von Eckardt, Germany’s ambassador to Mexico. Eckardt is to propose an alliance to Mexico, offering Texas, New Mexico and Arizona, territory they lost to the USA in the 19th Century. The Mexicans are also to be furnished with generous financial support.
A quirk of the current diplomatic arrangements is that German communications to its overseas embassies run through Britain. The Germans are no fools and are aware that the British are monitoring their communications. Zimmermann’s telegram is a sensitive one, so it is despatched in an encrypted form.
Unfortunately for the Germans, the British have cracked their diplomatic cyphers. Now Room 40, the secret British signal intelligence unit, sets to work on the telegram. Soon they will reveal its explosive secrets.
The Zimmermann Telegram (Wikipedia)