In the spring of 1917, despite two and a half years of fighting, the Allied and German forces on the western front were still deadlocked. The battle of the Somme had just drawn to a close with huge losses on both sides. The Germans decided to seek victory through a different route – by unrestricted U-boat warfare in the Atlantic, which meant using submarines to sink all merchant ships heading for the UK regardless of their nationality, in order to starve out the UK population.


Up to this point, the US had remained neutral – though it was supplying the Allied forces with large amounts of food and war materials from its factories. Thus there were significant numbers of US companies and ships trading with Britain. The concern for the Germans was that attacks on this trade might bring the US into the war.

On 17 January 1917, British intelligence intercepted the Zimmermann telegram, leading to one of the first occasions when a piece of SIGINT (intelligence gained by eavesdropping on an enemy’s coded communications) heavily influenced the course of world events.

The telegram was an internal diplomatic message sent in January 1917 from the German foreign secretary Arthur Zimmermann in Berlin, to the German Embassy in Mexico. It proposed a military alliance between Germany and Mexico, in the event of the United States entering the First World War in support of the Allies. When the contents of the telegram were made public in the US, it became a major factor in debates about whether the US should enter the war on the Allied side – which they did on 6 April 1917, just five weeks after the telegram’s publication.

Intercepted by ‘Room 40’

Although wireless (radio) was used to send messages in the First World War, the principal means of diplomatic communication was via telegrams sent on undersea cables. In 1914 the Allies cut many German cables, forcing them to communicate via the cables of other powers. German communications with the US, and other embassies in north and south America, were routed through the neutral US Embassy in Berlin, via their cable to Stockholm, and on across the Atlantic. Yet this cable actually passed through the UK and could be tapped by the British intelligence services.

The Germans relied on the fact that although the signals were on a ‘public’ cable, they were written in code, so the contents would remain secret. What they did not appreciate was that the British had already broken the codes they were using, and so any messages sent could be read. The Zimmermann message was passed to the British code-breaking unit in ‘Room 40’ of the Admiralty, where it was tackled by senior British code-breakers including Nigel de Grey and William Montgomery. Although they managed to understand the significance of the message very quickly, their understanding of the code was incomplete, so it took them several weeks of hard work to complete a full decryption.

The telegram instructed the German ambassador in Mexico, Heinrich von Eckardt, that if the United States appeared certain to enter the war, he was to approach the Mexican government with a proposal for military alliance with funding from Germany. The Mexicans were to be encouraged to invade the southern US with the aim of re-conquering those states that were formerly part of Mexico: Texas, Arizona, and New Mexico. Germany would support this effort with money and arms, and back Mexico’s territorial claims in any subsequent peace negotiations.

For Germany, an alliance with Mexico would simply be one of convenience. Mexico had suffered several years of war and political instability up to 1916, and Germany had already shown itself to be reluctant to involve itself in the country or support the current Mexican government. However, a stronger and friendly Mexico would be helpful in curbing the power of the US after the war. The US, on the other hand, regarded the Americas as their sphere of influence, and a rival power gaining a foothold in north America through alliance with their neighbour was highly undesirable.

The difficulty for the British Admiralty was that if they simply handed over the message to Washington, it would become clear that the British were tapping the US cable. If, however, they could acquire a copy from another source, this would provide a cover for why the British knew of the content of the message.

Sir Thomas Hohler had been first secretary of the British legation in Mexico City in September 1916. During that time Hohler had secretly arranged for copies of encoded messages from the German embassy in Washington to Mexico City (sent by commercial Western Union telegraph services) to be stolen, and passed to the British. Although he was no longer in his post in January 1917, this ‘arrangement’ continued, and so the British Embassy in Mexico was able to obtain a copy of the message on that leg of its journey. This had the added advantage that the message had been re-encrypted in Washington into a different German code, which was better understood by Room 40, so a more complete decryption could be created.

Influencing public opinion

Before the telegram was revealed publicly, Germany had waged ‘unrestricted submarine warfare’ between 4 February 1915 and 1 September 1915, and resumed from 1 February 1917. Several US ships were quickly sunk, and others were held in port for fear of being attacked. This led to a rise in anti-German, and pro-war feeling in the US. President Wilson was shown the Zimmermann message on 24 February, and released it to the press on 1 March. The wave of anti-German and anti-Mexican feeling grew in the US. Tensions with Mexico were already high; General John J Pershing had long been chasing the revolutionary Pancho Villa and carried out several cross-border raids with US forces.

At the same time, many Americans wished to avoid the conflict in Europe. Anti-British elements, particularly among German- and Irish-Americans, protested against involvement in the war. Since the public had been told (untruthfully) that the telegram had been stolen in a deciphered form in Mexico, the message was widely believed at first to be an elaborate forgery perpetrated by British intelligence.

While it was not the only factor in the US declaration of war, the telegram was certainly influential in the decision. It took the US some time to fully mobilise, and US troops played only a relatively small part in the fighting in 1918, but their economic support, and the knowledge that they were preparing to help, was a huge factor in the Allied victory. Once the US had formally thrown its weight behind the Allied cause, it was clear that the Germans were doomed to lose the war in the long run. They were forced to change their strategy, from trying to slowly wear out the British and French, to needing to bring the war to a successful conclusion in a shorter period. The failed German offensive of March 1918 was a symptom of this.

Meanwhile, the decryption of the telegram, and the work done in Room 40 more generally can be seen as the direct precursor of the formation of the Government Code and Cipher School in 1919. ‘GC&CS’ would go on to be based at Bletchley Park during the Second World War, and later formed the GCHQ of today. The political effect of the telegram contributed to the realisation of the power of codebreaking and signals intelligence in both war and peacetime diplomacy. As a consequence, when the Second World War broke out, Britain was already prepared to build on its experience to create a unique and war-winning SIGINT operation at Bletchley Park. This expertise, and the tradition of co-operation and intelligence sharing with the US, continues to form a key part of British defences to this day.

‘The Road to Bletchley Park’ exhibition, featuring the story of the Zimmermann telegram, can be found in Block C at Bletchley Park. For more information, visit www.bletchleypark.org.uk.


This article was first published by History Extra in April 2017.