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Room 40 : British Naval Intelligence 1914-18

The Zimmermann Telegram

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Intelligence Gathering Directed by Admiral Sir Reginald Hall Leading to the Involvement of the United States in World War I
Intelligence Gathering Directed by Admiral Sir Reginald Hall Leading to the Involvement of the United States in World War I
by Kevin Mulcahy

The events leading up to United States involvement in World War I were once closely guarded secrets. Since the end of the war, officials began publishing their memoirs, classified documents have been released, and researchers have uncovered information related to German intentions. It has been discovered that one piece of intercepted information may have provoked the United States to declare war against Germany. This information was obtained through the hard work, cunning, and sheer luck of a secret British intelligence operation known as Room 40.

Admiral Sir William Reginald Hall of the British Navy was instrumental in the operations of Room 40. It was in Room 40 that a top secret transmission sent from Germany via the United States to Mexico was decoded. It was also the tipping point that pushed the United States to enter World War I. This history changing document became known as the Zimmermann Telegram.

Before the methods of intelligence gathering used by Admiral Hall and Room 40 are discussed, it is important to understand the historical importance of the Zimmermann Telegram, and the information contained in the decoded message.

The importance of the message that was intended for Mexican eyes only was identified as information that needed to be shared with the United States and President Woodrow Wilson.

The message was intercepted by British intelligence and promptly decoded. The message stated that the Germans would begin unrestricted submarine attacks starting on February 1, 1917. This meant that any merchant ships observed in the war zone and not affiliated with the Germans would be attacked and sunk. The following is a copy of the text intended for Mexico:

“We intend to begin on the first of February unrestricted warfare.

We shall endeavor in spite of this to keep the U.S.A. neutral. In the event of this not succeeding we make Mexico a proposal of alliance with the following terms: Make war together. Make peace together.

We shall give generous financial support and an undertaking on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona. The settlement in detail is left to you.

You will inform the President of the above most secretly as soon as the outbreak of war with U.S.A. is certain, and add the suggestion that he should on his own initiative invite Japan to immediate adherence and at the same time mediate between Japan and ourselves.

Please call the President’s attention to the fact that the ruthless employment of our submarines now offers the prospect of compelling England in a few months to make peace. Zimmermann”
(The Code Breakers of Room 40, pg 141).

After consideration of the Telegram, Mexico wisely decided not to engage the United States in war along with Germany.

Although Germany promised monetary compensation and supplies, it would be nearly impossible for German ships to carry munitions to Mexico without being discovered by U.S. forces. Mexico had also expended a great deal of effort in building positive relationships with the United States and did not want to see those efforts go to waste. The final thought was that even if Mexico was successful in reclaiming lost territory, it would be nearly impossible to control the Americans living in what would then become Mexican territory.

Decoding the Zimmermann Telegram was an immense achievement on the part of Admiral Hall. Hall had the foresight to assemble a team of qualified intelligence personnel. Room 40 was filled with code breakers with the task of decoding German intelligence transmissions. Two senior code breakers were on duty when the Zimmermann Telegram arrived.

William Montgomery and Nigel de Gray are responsible for using the tools provided by Admiral Hall to decode the Zimmermann Telegram. Although it was expected that this telegram would contain mundane information, as countless telegrams in the past had, it was soon understood that this telegram was sent in German diplomatic code.

Montgomery and de Gray began to realize the importance as they painstakingly uncovered the text contained in the telegram. Nigel de Gray rushed the telegram to Admiral Hall once it was successfully decoded (The Zimmermann Telegram, pgs. 7-8). Admiral Hall forwarded the Zimmermann Telegram to the United States immediately after reading it.

This was the ultimate achievement of Room 40. Decoding the Zimmermann Telegram was the culmination of hard work and cooperation between countries.

German codes of the time were complicated and required code books to decode the information hidden in the secret messages. Several German code books were obtained through some very unlikely and almost unbelievable occurrences.

The British Committee of Imperial Defense realized that by cutting communication cables used by the Germans, they could severely limit the means of information transfer available to the Germans. A British ship was dispatched immediately after England declared war on Germany. The ship cut several transatlantic cables used for communication purposes by the Germans.

These actions cut off most wired communication between Germany and the rest of the world. Germany was forced to rely on wireless transmission of messages, and Room 40 capitalized on this weakness. Room 40 was able to intercept wireless messages and decode them with various code-breaking techniques they had acquired through different avenues.

The cutting of communication cables left only two forms of communication that were both easily intercepted. The mail system was the easiest system to infiltrate and the German government was aware of this. This forced the Germans to rely on the fairly new technology of wireless communication. The complex codes used by the German government would have been nearly impossible to decode if the code books they used were better secured.

One code book was discovered shortly after war was declared on Germany. The German merchant ship Hobart was intercepted off the coast of Australia. An Australian intelligence team masquerading as a “quarantine inspection team” boarded the vessel. The crew aboard the German ship were unaware that war was recently declared on Germany. They did not suspect that the team boarding their ship had hidden motives.

The captain of the Hobart went to his office to secure his copy of a German code book and the book was immediately seized by the Australian intelligence team. This book held the code used by the German High Sea Fleet and was known as the HVB code. A copy of this book was immediately sent to the British government and was quickly sent to Room 40 for analysis.

Before the HVB code book reached Room 40, a more secretive and important code book was obtained. This code was known as the SKM code and was given to British intelligence courtesy of the Russian government. The method of obtaining this book was the result of carelessness on the part of the German government.

On an uncharacteristically foggy day the German cruiser Magdeburg was conducting a mission off the coast of Russia. The ship was sent out even though it was underpowered.

The vessel was designed to be powered by three engines, but one malfunctioned and was removed. To make matters worse, the captain and crew of the ship were inexperienced. The fog and inexperience contributed to the Magdeburg crashing into an island off the Russian coast.

Russian cruisers discovered the Magdeburg shortly after the crash. Accounts of the events that transpired that day vary considerably. The most likely account is that many German sailors were being evacuated to another German ship as the Magdeburg was discovered by the Russian cruisers.

The Russians opened fire on the Magdeburg, preventing further evacuation of the German sailors. Captain Habenicht of the Magdeburg, as well as the remaining 57 sailors, were captured as prisoners of war. A search of the Magdeburg uncovered many secret documents including the SKM code book.

Some information published describes the Russians prying the code book from the arms of a dead German sailor floating in the sea (The Zimmermann Telegram, pg 15).

In later years the son of the Russian Ambassador to London released information stating that the code book was located in the chart house of the Magdeburg. This is the most likely manner in which the book was discovered because there was no evidence of water damage to the code book when it was later inspected.

The Russians immediately understood the importance of this code book. It is unclear why the Russians chose to provide the British with a copy of the valuable code book, but the fact that they did gave Room 40 one more key to unlock German codes.

Another code book was obtained in an equally unlikely and almost unbelievable manner. This code book was obtained by the British Royal Navy in a roundabout way. Officers in the British Navy were conducting surveillance because they received information, possibly from Room 40, that German destroyers were placing mines in strategic areas in the ocean.

The British surveillance team soon observed several German ships and immediately attacked. The vessels were sunk and the British ships returned to port. Unknown to British intelligence at the time, the captain of one of the German vessels gathered together several secret documents as well as a code book needed to decode the VB code and put them into a chest made of lead. The chest was thrown overboard from the deck of the sinking ship.

One month after the German ships were successfully destroyed a fishing vessel was trawling for fish. When the net was pulled up from the sea a lead chest was discovered among the fish.

The chest was turned over to the British government and then given to Admiral Hall in Room 40. Along with the papers that were extremely valuable to the British, the chest contained the VB code, which was used to decode the Imperial German Navy code (Room 40, pg. 7).

Another portion of code took more effort to obtain. A young university student working for the German government was convinced to steal the code and send it to the British. It is unclear if his mother or sister helped persuade the young man by the name of Alexander Szek, but a female relative sent him a letter urging him to assist the British.

Szek copied portions of the code and turned them over to a British intelligence agent. Shortly after the last piece of copied code was handed over, Szek went missing.

Theories of his disappearance include the possibility that the British disposed of him so the Germans would not discover that the code had been compromised. Another theory is that the Germans discovered the theft of the code and killed him.

One very important code book was discovered in the luggage of a captured enemy combatant. Wilhelm Wassmuss inadvertently delivered a German diplomatic code book to the British. This code was known as Code Number 13040. Wassmuss’ goal was to engage Persia into war in alliance with Germany. While spreading his message of war against the British, he was captured and sold to the British. Wassmuss escaped, but left his belongings behind. Inside one bag was Code Number 13040.

Admiral Hall received a voluminous amount of information from many different sources. He had all the tools necessary in place when the most important piece of coded information arrived at his intelligence installation. Room 40 intercepted the Zimmermann Telegram over three different communication routes.

The message was sent wirelessly to Mexico, over the Swedish Roundabout, and the American cable (The Zimmermann Telegram, pg. 147). The fact that Room 40 obtained the telegram from three different communication channels proved that Admiral Hall had nearly complete control of all messages sent by the Germans. Admiral Hall’s only dilemma was what to do with the highly sensitive information he received.

If Admiral Hall decided to divulge the contents of the telegram to the United States, U.S. ships would most likely be warned to avoid German occupied shipping lanes. German U-boats would immediately report the reduction of vessels in the area and would come to the realization that their codes had been compromised. German intelligence would immediately discontinue the use of the codes they once considered secret.

If German intelligence began using new codes, Room 40 would become obsolete. Admiral Hall weighed the costs and benefits of divulging the Zimmermann Telegram to the United States. He came to the conclusion that the benefits of sending the information to the United States greatly outweighed the risk of being discovered by German intelligence.

He knew he had to develop a fictitious story of how the telegram was discovered that would be believed by German intelligence. Admiral Hall sent an agent to Mexico to intercept a copy of the Zimmermann Telegram. This would give the British a believable story to tell the United States how it was obtained. The story proved to be effective. The Germans believed that a double-agent working in the German embassy was behind the theft of the Zimmermann Telegram, although a double-agent was never discovered.

President Woodrow Wilson knew that he had to act on the information presented to him.

To add credibility to the document, President Wilson needed to obtain the original document sent through Western Union. Undersecretary of State Frank Polk was tasked with convincing Western Union to produce the document (Encyclopedia of Intelligence and Counterintelligence).

The Zimmermann Telegram was one of the main reasons that the United States declared war on Germany. Germany sank many ships with Americans aboard. Although at least one of the destroyed vessels may have been carrying munitions destined for allied forces, the American public was enraged by the German attacks. Two months after the Zimmermann Telegram was delivered to the United States, President Woodrow Wilson formally entered World War I.

Before the Zimmermann Telegram reached President Wilson, the United States was committed to remaining neutral.

President Wilson had no intentions of engaging Germany in war. Admiral Hall knew that this document would likely compel the United States to enter the war (Encyclopedia of Intelligence and Counterintelligence).

On March 17, 1918, the American Ambassador in London sent President Woodrow Wilson a letter describing the importance of Admiral Hall. Ambassador Page wrote, “Such eyes as the man has! My lord! I do study these men here most diligently who have this vast and appalling War Job. There are most uncommon creatures among them – men about whom our great-grandchildren will read in school histories; but, of them all, the most extraordinary is this naval officer – of whom, probably, they’ll never hear” (Eyes of the Navy, pg. VI).

Admiral Hall built a complex network capable of changing the course of history.

He equipped Room 40 with the personnel and equipment necessary to intercept and intelligently interpret German information. The successful strategies employed by Admiral Hall continue to be studied by the intelligence community.

After retiring from his service in British intelligence, a biography of Admiral Hall was released. The biography was written by Admiral William James, who took over Room 40 after Admiral Hall’s retirement. Admiral James highlighted the importance of Room 40 and Admiral Hall’s contributions that led to the involvement of the United States in World War I (Journal of Military History, pg. 939).

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Copyright © 2015 Kevin Mulcahy

Written by Kevin Mulcahy. If you have questions or comments on this article, please contact Kevin Mulcahy at:

About the author:
Kevin Mulcahy is a military history enthusiast as well as a Physical Security specialist. He is employed in a security leadership role for a corporation encompassing 1.4 million square feet. He has multiple security and safety certifications. In his spare time he enjoys outings with his family and labrador retriever.

Published online: 07/26/2015.

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Beesly, P. (1982). Room 40: British naval intelligence: 1914-1918. New York, NY: Harcourt Brace Jovanovich.

James, W. (1956). The Eyes of the navy: a biographical study of Admiral Sir Reginald Hall. London: Methuen & Co. Ltd., London.

Sharpe, M.E. (2005). Zimmerman Telegram. (2005). Encyclopedia of intelligence & counterintelligence. Armonk, NY.

The Journal of Military History 63 (October 1999): 939-51. Krakow, Poland: The Enigma Press.

Tuchman, B. (1986). The Zimmermann telegram. New York, NY: Ballantine Books.

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