If you are a history buff like me, you probably know the story already: in early January 1917, the German Foreign Secretary, Arthur Zimmermann, sent an encrypted telegram to the country’s ambassador in Mexico. The telegram contained a secret scheme for a military alliance between Germany and Mexico, in the event that the USA would enter the war on the side of Britain and the Entente powers.

Equally well known is the dramatic twist: the telegram was intercepted by British intelligence officers, who cracked the code and leaked the content to President Woodrow Wilson’s administration. A few days later, when the decrypt was published by Associated Press, it caused what we today would call a “shit-storm” against Germany in US media. And on 2 April 1917, when President Wilson gave his war address to Congress, he explicitly invoked the Zimmermann Telegram as proof of Germany’s treacherous foreign policy; of the Kaiser’s intentions “to stir up enemies against us at our very doors”.

A translated decrypt of the infamous telegram, circulating in US media during the critical months leading up to the US entry into World War One.

So the Zimmermann Telegram was a critical factor in the complicated process that eventually led to the US entry into World War One. Many historians have therefore argued that it changed the course of history. At the very least, it is one of the most iconic telegrams in world history.

What is less known, however, is that the infamous note was transmitted via the Main Telegraph Station in Copenhagen. It was wired from Berlin to Copenhagen on 16 January 1917 – and then on to London through the submarine telegraph cable from Søndervig in Denmark to Newcastle in the UK. The telegram reached the German embassy in Washington on 19 January, from whence it was forwarded to Mexico City. But by then its content had already been uncovered by the British Admiralty’s code-breaking unit “Room 40”.

I have for a long time nourished a naïve dream about finding some trace of this legendary piece in Danish telegraph archives: an entry in a log, a number in ledger or a secret note from an even more secret Danish intelligence unit about a new, unbreakable German code. Finding something along these lines would definitely be quite a prize for my museum, which besides being the only proper communications museum in Denmark has been located since 1998 in the same historical building that once housed the Main Telegraph Station in Copenhagen. (We are currently about to move, however, to the stately postal office in the borough of Østerbro, but that is another story that you can read about in other postings on this blog.)

Yet there is also another dimension to my dream about Zimmermann. If I found an impression of this dramatic piece of world history in Danish documents, I could use it in my efforts to tell an alternate history of Denmark and Scandinavia during World War One; a version that does not focus on the neutral and unaffected “Lilliput country” angle but on the largely forgotten fact that Denmark was a hub in the global telegraph networks and turned into a hive of propaganda, espionage and other kinds of information warfare during the Great War. It is not for nothing that the war years 1914–1918 are remembered as “the Golden Age” in the chronicles of the Danish Telegraph Service.

So what about my quest for the Zimmermann Telegram? Have I exhumed any solid material about its trail through the Danish networks? Well, there are still no direct hits, I have to admit, but I have dug up a veritable smorgasbord of documents that testify to the strategic importance of Denmark and its telecommunications networks during World War I. For instance, I really did find that ultra-secret Danish intelligence unit, hidden in a humble looking back-office at the Central Telegraph Station. But I will save that story for another posting.

Today I would like to share another discovery – a story from the vaults that least indirectly relates to the intricate history of the Zimmermann affair. It regards a complaint submitted on 31 May 1915 to the Danish Foreign Ministry by the British ambassador in Denmark:

His Majesty’s Minister has been informed by His Majesty’s Principal Secretary of State that the United States Embassy in London state that the Danish Telegraph Authorities have stopped cypher messages from the United States Ambassador at Berlin to the United States Embassy in London on the ground that they had never been informed that His Majesty’s Government allowed cypher messages from Diplomatic Missions.

So Britain mediated on America’s behalf in a case regarding some encrypted state telegrams, which allegedly had vanished en route from Berlin to London via Danish cables. Most probably, the ambassador suggested, the whole thing was based on a misunderstanding: the British censors were usually pretty strict and forbidding about “messages in cypher”, but correspondence involving the US State Department and American embassies were explicitly exempted from any restrictions.

The British diplomatic document, a so- called "Aide Mémoire", submitted to the Danish Foreign Ministry on 31 May 1915.

But why is this relevant for our understanding of the Zimmermann affair? Well, it is a bit complicated – and delicate. One of the juiciest quirks to the story is that the ill-fated German telegram was sent along the US State Department route – from the US embassy in Berlin via Copenhagen and London to Washington. This sounds odd, but it was politically quite logical at the time. Due to British hardware attacks in the beginning of the war, Germany was practically cut-off from any direct access to the global telegraph networks. Therefore, the Kaiser and his ministers had to rely on the diplomatic services of friendly neutrals such as Sweden – and, ironically, the USA. A great number of encrypted messages from the German Foreign Ministry in Berlin to its embassy in Washington actually crossed the Atlantic Ocean in the guise of US diplomatic cables. The traffic became particularly intense after the German sinking of the ocean-liner Lusitania in May 1915, when direct means of communication between the two governments became imperative.

What neither the Germans nor the Americans knew, however, was that Britain was fully aware of this arrangement – and that British intelligence officers actively eavesdropped on all diplomatic communication between Washington and Berlin (as well as on all conversations along the parallel so-called “Swedish Roundabout”). The British interception of the Zimmermann Telegram was thus the direct result of this continuous and absolutely ultra-secret monitoring of US diplomatic correspondence.

This is where the British complaint to the Danish Foreign Ministry becomes interesting. The note was delivered some three weeks after the sinking of the Lusitania, i.e. during a period when the diplomatic correspondence between Berlin and Washington was particularly tense – and dramatically increasing. Could it be that this intervention into a matter that primarily concerned Denmark and the US not only should be read as a British favor to an important, Entente-friendly neutral, but also as an attempt to secure the US State Department route as a source of secret intelligence? A sudden disruption of the covert correspondence between Berlin and Washington would definitely have deprived the British intelligence sector of a valuable peephole into German strategic planning and thinking. Take the Zimmermann Telegram, for instance. It is quite possible that the proposal to the Mexican government would have remained uncovered if the message had been sent through other channels, for instance a human courier.

So what happened to the missing US telegrams, which allegedly had been stopped by Danish authorities? They were actually never located again, not in Denmark at least. The case was forwarded to the head of the Danish Telegraph Service, who initiated a prompt investigation into the matter. And according to his internal report to the Ministry for Public Works, “no cypher telegram from the American ambassador in Berlin has been held back here”. He concluded that the whole case was a big misunderstanding, as he had been in touch with American embassy personnel in Copenhagen, who claimed to have no knowledge of any Danish telegraph stations that “refused to accept cypher telegrams to the American embassy in London.” Thus, about a week after the British protest, the Danish Foreign Ministry replied in a note to the British ambassador, sir Henry Crofton Lowther, that “no messages from the United States Ambassador in Berlin has been stopped in this country.”

Whether this is the only version of the truth in this case is an open question. The Danish High Command, as previously mentioned, had a secret office for signals intelligence at the Main Telegraph Station in Copenhagen, and I have dug up a number of suspicious telegrams that were made to “discreetly disappear” in Denmark during World War One. However, I have not found any more complaints about Danish obstacles along the US State Department route, so the British could probably quite easily carry on with their secret eavesdropping on the cross-Atlantic flows of information.