Communications Surveillance during World War I

The dark and stormy circumstances of World War I, 1914–1918, prompted the development of government-backed regimes for mass surveillance of electric and postal communications across Europe. The transnational flows of information, which had expanded rapidly since the mid-nineteenth century, turned into a security risk at the outbreak of hostilities, as they were linked to escalating government fears of enemy propaganda, information leaks and espionage. Moreover, the war years saw the birth of modern signals intelligence, as demonstrated by the famous case of the ill-fated German Zimmermann Telegram.

Two young telegraphers at the Main Telegraph Station in Copenhagen, app. 1915, deciphering a German telegram for a secret military intelligence unit called “Kystcentralen”

At the outbreak of World War I in August 1914, the transnational networks of communication, which connected the European countries with each other and the globe, turned into a security risk to the national governments. In belligerent as well as neutral states, the years 1914–1918 saw the rise of government-backed systems for mass-surveillance of telecommunications and postal mail. 

Already on August 1, when Germany declared war on Russia and France, the Telegraph Directory in neutral Denmark issued a secret order that all foreign-bound telegrams containing “sensational and false messages regarding Danish conditions or public moods” had to be intercepted by the telegraph staff. On the same day in Germany, a system for military censorship was established for all kinds of postal and electric communication. Encryption of private telegrams became forbidden and the telephone lines from Germany to the outside world were disconnected.  The domestic telephone networks would remain open though, also for private communication. Yet, it was explicitly forbidden throughout the war to speak in languages other than German on the German telephone lines.

Similar restrictions and control mechanisms were established throughout the European communication space, from Britain and France to Russia and the Scandinavian countries, as the border-crossing flows of information were linked to government fears of propaganda, information leaks and espionage. 

Public opinion and propaganda

An important objective of the new surveillance regimes was to thwart the influence of foreign propaganda on people and media at the so-called “home-fronts” of Europe, i.e. within the national borders of the countries. Growing literacy and a burgeoning press laid the ground for innovative and more subtle forms of propaganda that operated through the dissemination of news. The telegraph became a tool of wartime propaganda, so did the three big European news agencies, Reuters (Britain), Agence Havas (France) and Wolffs Telegraphisches Bureau (Germany), which spread propaganda disguised as news reporting.

One of the tasks of the telegraph censors was to hinder such news items from reaching the public by closely monitoring incoming press telegrams and seizing everything that appeared suspicious or biased to the ‘wrong side’. The evaluation of the content was particularly difficult in the neutral countries, which had to balance between the belligerents and their strategic interests.

Yet out-going press telegrams were also scrupulously checked before they could enter the transnational networks. The monitoring of war reporters and other journalists was crucial for the new kinds of informational warfare that developed during World War I, to which the shaping and manipulation of public opinion—domestically as well as abroad—became a number one priority.

Counterintelligence and fear of espionage

The monitoring of press telegrams was also linked to provisions for military secrecy and counterintelligence, i.e. the protection against espionage and other forms of foreign intelligence activities. There was a fear that strategically sensitive information could be leaked through the press, either accidently through carelessness or willingly as spying for enemy actors. Yet, although many governments and military authorities were particularly suspicious of the press, the struggle against this potential security risk was not limited to the global circulation of news. Both private and business telegrams were checked for sensitive content, along with outgoing postal mail. All kinds of encryption and code-books were forbidden, a provision that caused problems for the business sector, whose communication in trade-codes or simple digits—signifying quantity, dates and prices—was severely complicated. Only governments and their diplomatic representatives were allowed to utilize coded language in their communication.

Domestic communications were also being monitored, but usually in a more irregular manner than border-crossing flows of information. Domestic post in wartime Germany was only controlled during periods when the authorities considered the security situation “extreme”, while all mail going out of the country was opened and examined throughout the war, also chemically to look for invisible ink. Moreover, in countries that still had operating international telephone lines, as in the Scandinavian region, attempts were being made at telephonic censorship. Telephone operators were instructed to eavesdrop on conversations and to disconnect any callers that were deemed to pass on strategically sensitive information.

Wireless radio communication, which in 1914 was a technology in its infancy, was considered particularly treacherous. Radio-waves crossed borders without discrimination and were extremely difficult to manipulate and control. Most of the countries that before the war had experimented with wireless communication prohibited private usage and ownership at the outbreak of hostilities. Private radio-sets were confiscated and wireless stations appropriated by the state.

For the same reason, private keeping of homing pigeons was forbidden throughout Europe. The military made massive use of message-carrying birds, as well as dogs and other animals, but in private hands they were rendered a connotation of espionage and treacherous attempts at avoiding censorship.

The rise of modern signals intelligence

Another aim of wartime surveillance was the collection of intelligence on enemy states and other interesting actors. The breaking of telegram codes used by foreign governments evolved into an important strategic tool and many historians argue that World War I saw the birth of modern signals intelligence.

At the onset of the war, the most efficient units for communications-based intelligence were those in Russia, France and Austria-Hungary. A case in point is the French cabinet noir, which already in the years before the war had managed to crack the diplomatic codes of Britain, Germany, the Ottoman Empire and a number of other rival states. The French could thereby secretly follow postal and telegraphic communication between the governments concerned and their diplomats in France.

Yet, during the course of World War I, Britain became the leading actor in the field of signals intelligence. This was largely due to her crucial position at the hub of global telecommunications networks and the fact that she owned a substantial part of the very cables that carried information across borders. Nothing illustrates this combined infrastructural and informational power better than the case of the Zimmermann Telegram. The ill-fated note was drafted by the German Foreign Secretary, Arthur Zimmermann (1864-1940), in early January 1917, a few weeks before the Germans would resume their strategy of unrestricted U-boat warfare. It contained a secret scheme for a military alliance between Germany and Mexico, in the event that the controversial submarine strategy would provoke the USA to enter the war on the side of the Entente powers. The encrypted telegram was wired from Berlin to Copenhagen on 16 January 1917 and then on to London through the submarine cable from Søndervig in Denmark to Newcastle in Britain. After crossing the Atlantic via British cables, it reached the German embassy in Washington on 19 January. From there it was forwarded in a simpler kind of code to the German ambassador in Mexico City, yet by this point the content had already been exposed by the British Admiralty’s code-breaking unit “Room 40”. The German plan was leaked to President Woodrow Wilson’s administration and US media published a translated version of the telegram, which caused a public outcry against Germany. When Wilson gave his war address to the American Congress on 2 April 1917, he invoked the Zimmermann Telegram as proof of how dangerous Germany had become to the USA. Thus, this crucial piece of intelligence—collected from the transnational flows of information— became a factor in the complicated process that eventually led to the US entry into World War I.

To quote from this article

Andreas Marklund , « Communications Surveillance during World War I  », Encyclopédie d'histoire numérique de l'Europe [online], ISSN 2677-6588, published on 26/06/20 , consulted on 18/05/2024. Permalink :


Boghardt Thomas, The Zimmermann Telegram. Intelligence, Diplomacy, and America’s Entry into World War 1, (Annapolis: Maryland, Naval Institute Press, 2012).

Jander Thomas & Didczuneit Veit, Netze des Krieges. Kommunikation 1914–18, (Königswinter, Brandenburgisches Verlagshaus, 2014).

Laurent Sebastian-Yves, Le Secret De L’état. Surveiller Protéger Informer xviiexxe Siècles, (Paris, Archives Nationales, 2015).

Marklund Andreas, “Listening for the State: Censoring Communications in Scandinavia during World War One”, History & Technology, 2016.