The Z gram intercept: greatest cryptography coup of World War I

Spokesman Magazine, July, 2004 by Dennis Casey

Early in the morning on January 17, 1917, the duty officer at British Naval Intelligence opened a pneumatic tube, examined the wireless German intercept it contained and concluded that it was nothing of obvious significance. Indeed, to the duty officer it looked like thousands of other messages that had been intercepted since the beginning of the war some thirty months ago. To two others on duty at Whitehall that morning, the intercept would soon take on another meaning.

Secretly assigned to cryptographic work, Reverend William Montgomery, a gray-haired scholar of forty-six, and Nigel de Grey, a thirty-one year old publisher on loan from the firm of William Heinemann, were the first to work with the intercept. On initial examination, DeGrey found the telegram to consist of rows of numerals arranged in groups of four and five with a sprinkling of three-number groups. The numbers gave absolutely no hint whatsoever of any hidden message. DeGrey noticed at the outset, however, that the message was unusually long and contained over a thousand groups of numbers. He also noted that the top group of numbers in the message, 13042, amounted to a variant of 13040, the title number of the German diplomatic code. Turning then to a reconstructed German codebook for code 13040, the decoders first tried to come up with the signature. Both DeGrey and Montgomery recognized that the signature in this case used high numbers, 97556, usually indicated someone highly placed in the German government. They soon discovered that Arthur Zimmermann, the German Foreign Secretary, had likely sent the message.

DeGrey and Montgomery then began work to determine the recipient of the message. The key jumped out when they discovered that in the address portion of the message there appeared the words "For Your Excellency's personal information. Since the message had been sent to the German Embassy in Washington, this meant the German Ambassador, Count Johann von Bernstorff was the recipient.

As the two continued with their analysis, they both felt that the German communication seemed very ordinary. This changed when they determined that the number sequence for Mexico appeared. With their interest now alerted, they then came up with the words alliance and later Japan. In fact the decoders came up with the phrase "us and Japan." Both questioned what this could mean. Was Japan about to switch sides in the war? After several more hours of work, an intelligible version of what had been said in the intercept became clear.

The intercept fell into two parts. The first and longest addressed to von Bernstorff informed him of Germany's intention to resume unrestricted submarine warfare on February 1 meaning that the Germans would sink neutral as well as enemy merchant shipping located in the war zones. Bernstorff was instructed not to deliver the message to the United States until February 1. The second part of the message read, "For Your Excellency's personal information and to be handed on to the Imperial Minister in Mexico by a safe route." The still not completely processed message stated that if Germany was not successful in keeping the United States neutral, then Germany proposed an alliance with Mexico for the joint conduct of the war and its conclusion.

DeGrey and Montgomery essentially were both astounded with their find. The intercept, they felt, would propel America out of its neutrality. That neutrality President Woodrow Wilson had maintained for several years using every diplomatic means conceivable to bring an end to the war and to prevent the Germans from resorting to submarine warfare. Yet there was a portion of the intercept that the two decoders had been unable to reconstruct. After literally weeks of effort they produced Germany's promise to assist Mexico "to regain by conquest her lost territories in Texas, Arizona, and New Mexico." DeGrey and Montgomery now concluded that if their work had produced potentially explosive results up to this point, this disclosure would further enhance the potential influence of Zimmermann's remarks. They decided to call in Admiral Sir William Reginald Hall, the Director of British Naval Intelligence. Upon reviewing the intercept, Hall recognized immediately that he held in his hands a document that could either produce a miracle, get America to enter the war on the allied side, or it could cut off the source of allied supplies and convince the Americans to remain neutral. Admiral Hall understood that Zimmermann was counting on the fact that if the Americans entered the war on the side of the allies, then he wanted to do something to keep the Americans busy on their side of the Atlantic, in this case dealing with potentially both Japan and Mexico as enemies. He also felt that Zimmermann was banking on the possibility that German submarines could sink ships faster than the Americans could build them and send them to Britain loaded with supplies. But how could Admiral Hall reveal to the Americans the content of the Zimmermann intercept without at the same time revealing how the information had been obtained? Further, if the Germans discovered their code had been broken, they would select another and an extremely valuable source of intelligence would dry up. Hall concluded he could not risk disclosure.