Last Updated: 02 Dec 2015
Britain’s Signals Intelligence effort essentially dates from the beginning of World War I

A number of radio intercept stations were created during World War I, and an increasing number of cryptanalysts, linguists and radio traffic analysts enjoyed considerable success in decrypting messages sent by Germany and its allies and in disseminating this intelligence to where it was needed.

The Zimmermann Telegram

The most famous Sigint report of World War I came from the decryption of a telegram sent by the German Foreign Minister, Count Zimmermann, in early 1917. It stated that as Germany was to undertake unrestricted submarine warfare against vessels (including neutral American ones) trading with the British, which might bring the USA into the War on the British side, Mexico would be rewarded with the recovery of its territories of Arizona, New Mexico and Texas if it joined the conflict. Release of the Zimmermann Telegram to the US authorities was one of the deciding factors in the USA joining the War on the side of the Allies.


Zimmer Coded Landscape

The Government Code and Cypher School

Sigint’s success in World War I and the interest of politicians such as Lloyd George, Lord Curzon, and Winston Churchill in Sigint reporting, led to the creation of a peacetime organisation in the Admiralty called the Government Code and Cypher School (GC&CS). It was initially a small organisation of 30 cryptanalysts and a similar number of support staff. Its job was “Construction, Destruction and Instruction”: providing advice on the security of British governmental codes and ciphers; the study of the methods of encryption used by foreign powers; and the training of British officials in the use of secure communications.

In 1922, the School was transferred from Admiralty control to the Foreign Office. It came under the administrative control of ‘C’, the Chief of the Secret Intelligence Service (SIS), though it remained completely autonomous of SIS in terms of what it collected and reported, and how it developed its capabilities. Naval, Military and Air Sections were added in the following years, and, after the Italian invasion of Abyssinia in 1935 and the start of the Spanish Civil War in 1936, GC&CS began to expand as the slide towards war began, and developed closer cooperation with naval and military collection sites worldwide, and, from 1938 onwards, with the Dominions. Most importantly, contact made with Poland and France led to a Conference in July 1939 at which the Poles shared all of the progress they had made against the German Army Enigma.