Room 40

Room 40 was the name of the British Admiralty’s codebreaking section during the First World War, taken from the number of a room in the Admiralty Old Building off Whitehall. Its cryptographers read German naval messages and, in time, diplomatic ones as well. Thanks to them the Royal Navy often knew in advance when the German fleet was putting to sea, and in 1917 the British learnt of the Zimmermann telegram.

A professor and three codebooks

In the first days of the war the British cut Germany’s transatlantic cables, and the Germans had to send by wireless or through the cables of neutral countries. Intercepted messages began to reach the Admiralty, but no one could read them. The Director of Naval Intelligence, Rear-Admiral Henry Oliver, handed them to his friend Alfred Ewing (PL), an engineer and Director of Naval Education who dabbled in ciphers as a hobby. Ewing gathered German-speaking teachers from the naval colleges, and later scientists, translators and publishers too. None of them knew anything about codebreaking, but all of them could keep a secret.

The breakthrough came from captures. The Russians found a signal book on the wreck of the cruiser Magdeburg, and on 13 October 1914 one copy reached the hands of the First Lord of the Admiralty, Winston Churchill. At the end of October a copy arrived in London of the code with which the German navy communicated with merchant ships, seized by the Australians on 11 August from the steamer Hobart, and on 30 November a British trawler fished up a chest from a German torpedo boat sunk off Texel, containing the code used by naval attachés and senior commanders. On 6 November 1914 the section moved into room 40. In the autumn Captain William Reginald Hall became Director of the Intelligence Division, and in May 1917 he took over direct control from Ewing.

War in the North Sea

The German fleet reported the positions of its ships by wireless every day, and from 1915 direction-finding stations on the coast made it possible to track U-boats as well. Any change in the usual rhythm heralded an operation. That is how the British learnt that the battlecruisers were putting to sea before the bombardment of Scarborough in December 1914 and before the Battle of Dogger Bank in January 1915. On 30 May 1916, forewarned by Room 40, John Jellicoe took the Grand Fleet to sea before the Germans had left harbour, which opened the way to the Battle of Jutland.

This knowledge, however, was used less well than it could have been. The Admiralty guarded the secret so closely that, with few exceptions, messages based on the codebreakers’ work were approved personally by Oliver, and commanders at sea received little information, often late. Nor did the codebreakers have any say in the conclusions drawn from their work. Before the Scarborough raid they did not know that the whole High Seas Fleet had put to sea, and at noon on 31 May 1916 Jellicoe received a mistaken message from the Admiralty that the German flagship was still in harbour.

The Zimmermann telegram

From 1915 Room 40 also read German diplomatic correspondence. Deprived of their own cables, the Germans used other countries’, and these passed through Britain. On 16 January 1917 the codebreakers intercepted a message from Arthur Zimmermann in which Germany proposed an alliance to Mexico against the United States, and the next day Nigel de Grey brought Hall a first reading. Hall held back so as not to reveal that the British were also reading American messages. Only after a copy of the telegram had been obtained from the telegraph office in Mexico did Arthur Balfour hand it to the American ambassador on 23 February. After the publication of the text and the resumption of unrestricted submarine warfare, the United States declared war on Germany. The historian of cryptology David Kahn held that no other single cryptanalysis had had such enormous consequences.

By one estimate Room 40 decrypted about 15,000 German messages during the war, and at its peak, according to one account, it employed some 800 wireless operators and several dozen cryptographers and other specialists. In 1919 it was merged with the army’s signals intelligence into the Government Code and Cypher School, which during the Second World War worked at Bletchley Park (PL) and later gave rise to GCHQ.