In the history of cryptanalysis, Room 40, also known as 40 O.B. (Old
Building) (latterly NID25) was the section in the British Admiralty
most identified with the British cryptanalysis effort during the First
World War, in particular the interception and decoding of the
Zimmermann Telegram which played a role in bringing the United States
into the War.
Room 40 was a group formed in October 1914, shortly after the start of
the war. Admiral Oliver, the Director of Naval Intelligence, gave
intercepts from the German radio station at Nauen, near Berlin, to
Director of Naval Education Alfred Ewing, who constructed ciphers as a
hobby. Ewing recruited civilians such as William Montgomery, a
translator of theological works from German, and Nigel de Grey, a
The basis of
Room 40 operations evolved around a German naval
codebook, the Signalbuch der Kaiserlichen Marine (SKM), and around
maps (containing coded squares), which Britain's Russian allies had
passed on to the Admiralty. The Russians had seized this material from
the German cruiser
SMS Magdeburg when it ran aground off the Estonian
coast on 26 August 1914. The Russians recovered two of the four copies
that the warship had carried; they retained one and passed the other
to the British.
In October 1914 the British also obtained the Imperial German Navy's
Handelsschiffsverkehrsbuch (HVB), a codebook used by German naval
warships, merchantmen, naval zeppelins and U-Boats: the Royal
Australian Navy seized a copy from the Australian-German steamer
Hobart on 11 October. On 30 November a British trawler recovered a
safe from the sunken German destroyer S-119, in which was found the
Verkehrsbuch (VB), the code used by the Germans to communicate with
naval attachés, embassies and warships overseas.
In March 1915 a British detachment impounded the luggage of Wilhelm
Wassmuss, a German agent in Persia and shipped it, unopened, to
London, where the Director of Naval Intelligence, Admiral Sir William
Reginald Hall discovered that it contained the German Diplomatic Code
Book, Code No. 13040.
The function of the
Room 40 program was compromised by the Admiralty's
insistence upon interpreting
Room 40 information in its own way.[need
quotation to verify]
Room 40 operators were permitted to decrypt but
not to interpret the information they acquired.
The section retained "Room 40" as its informal name even though it
expanded during the war and moved into other offices. It was estimated
by Westwood in 2009 that
Room 40 decrypted around 15,000 German
communications, the section being provided with copies of all
intercepted communications traffic, including wireless and telegraph
traffic. Alfred Ewing directed
Room 40 until May 1917, when direct
control passed to Captain (later Admiral) Reginald 'Blinker' Hall,
assisted by William Milbourne James.
2.1 Capture of the SKM codebook
2.2 Capture of the HVB codebook
2.3 Capture of the VB codebook
2.4 Room 40
2.5 Signals interception and direction finding
3 Zimmermann Telegram
5 Merger with Military Intelligence (MI)
8 External links
In 1911, a sub-committee of the
Committee of Imperial Defence
Committee of Imperial Defence on cable
communications concluded that in the event of war with Germany,
German-owned submarine cables should be destroyed. In the early hours
of 5 August 1914, the cable ship Alert located and cut Germany's five
trans-Atlantic cables, which ran down the English Channel. Soon after,
the six cables running between Britain and Germany were cut. As an
immediate consequence, there was a significant increase in cable
messages sent via cables belonging to other countries, and cables sent
by wireless. These could now be intercepted, but codes and ciphers
were naturally used to hide the meaning of the messages, and neither
Britain nor Germany had any established organisations to decode and
interpret the messages. At the start of the war, the navy had only one
wireless station for intercepting messages, at Stockton. However,
installations belonging to the Post Office and the Marconi Company, as
well as private individuals who had access to radio equipment, began
recording messages from Germany.
Intercepted messages began to arrive at the
division, but no one knew what to do with them. Rear-Admiral Henry
Oliver had been appointed Director of the Intelligence division in
1913. In August, 1914, his department was fully occupied with the war
and no-one had experience of code breaking. Instead he turned to a
friend, Sir Alfred Ewing, the Director of Naval Education (DNE), who
previously had been a professor of engineering with a knowledge of
radio communications and who he knew had an interest in ciphers. It
was not felt that education would be a priority during the expected
few months duration of the war, so Ewing was asked to set up a group
for decoding messages. Ewing initially turned to staff of the naval
colleges Osborne and Dartmouth, who were currently available, due both
to the school holidays and to naval students having been sent on
Alastair Denniston had been teaching German but later
became second in charge of Room 40, then becoming Chief of its
successor after the First World War, the Government Code and Cypher
School (located at
Bletchley Park during the Second World War).
Others from the schools worked temporarily for
Room 40 until the start
of the new term at the end of September. These included Charles
Godfrey, the Headmaster of Osborne (whose brother became head of naval
Intelligence during the Second World War), two Naval instructors,
Parish and Curtiss, and scientist and mathematician Professor
Henderson from Greenwich Naval College. Volunteers had to work at code
breaking alongside their normal duties, the whole organisation
operating from Ewing's ordinary office where code breakers had to hide
in his secretary's room whenever there were visitors concerning the
ordinary duties of the DNE. Two other early recruits were R. D.
Norton, who had worked for the Foreign Office, and Richard Herschell,
who was a linguist, an expert on Persia and an Oxford graduate. None
of the recruits knew anything about code breaking but were chosen for
knowledge of German and certainty they could keep the matter
A similar organisation had begun in the Military Intelligence
department of the War Office, which become known as MI1b, and Colonel
Macdonagh proposed that the two organisations should work together.
Little success was achieved except to organise a system for collecting
and filing messages until the French obtained copies of German
military ciphers. The two organisations operated in parallel, decoding
messages concerning the Western Front. A friend of Ewing's, a
barrister by the name of Russell Clarke, plus a friend of his, Colonel
Hippisley, approached Ewing to explain that they had been intercepting
German messages. Ewing arranged for them to operate from the
coastguard station at
Hunstanton in Norfolk, where they were joined by
Leslie Lambert (later becoming known as a BBC
broadcaster under the name A. J. Alan).
Hunstanton and Stockton formed
the core of the interception service (known as 'Y' service), together
with the Post Office and Marconi stations, which grew rapidly to the
point it could intercept almost all official German messages. At the
end of September, the volunteer schoolmasters returned to other
duties, except for Denniston; but without a means to decode German
naval messages there was little specifically naval work to do.
Capture of the SKM codebook
SMS Magdeburg aground off Odensholm
The first breakthrough for
Room 40 came with the capture of the
Signalbuch der Kaiserlichen Marine (SKM) from the German light cruiser
SMS Magdeburg. Two light cruisers, Magdeburg and SMS Augsburg, and a
group of destroyers all commanded by Rear-Admiral Behring were
carrying out a reconnaissance of the Gulf of Finland, when the ships
became separated in fog. Magdeburg ran aground on the island of
Odensholm off the coast of Russian-controlled Estonia. The ship could
not be re-floated so the crew was to be taken on board by the
destroyer SMS V26. The commander, Korvettenkapitän Habenicht prepared
to blow up the ship after it had been evacuated but the fog began to
clear and two Russian cruisers Pallada and Bogatyr approached and
opened fire. The demolition charges were set off prematurely, causing
injuries amongst the crew still on board and before secret papers
could be transferred to the destroyer or disposed of. Habenicht and
fifty seven of his crew were captured by the Russians.
Exactly what happened to the papers is not clear. The ship carried
more than one copy of the SKM codebook and copy number 151 was passed
to the British. The German account is that most of the secret papers
were thrown overboard, but the British copy was undamaged and was
reportedly found in the charthouse. The current key was also needed in
order to use the codebook. A gridded chart of the Baltic, the ship's
log and war diaries were all also recovered. Copies numbered 145 and
974 of the SKM were retained by the Russians while HMS Theseus
was dispatched from
Scapa Flow to Alexandrovosk in order to collect
the copy offered to the British. Although she arrived on 7 September,
due to mix-ups she did not depart until 30 September and returned to
Scapa with Captain Kredoff, Commander Smirnoff and the documents on 10
October. The books were formally handed over to the First Lord,
Winston Churchill, on 13 October.
The SKM by itself was incomplete as a means of decoding messages,
since they were normally enciphered as well as coded and those that
could be understood were mostly weather reports. Fleet paymaster C. J.
E. Rotter, a German expert from the naval intelligence division, was
tasked with using the SKM codebook to interpret intercepted messages,
most of which decoded as nonsense since initially it was not
appreciated that they were also enciphered. An entry into solving the
problem was found from a series of messages transmitted from the
German Norddeich transmitter, which were all numbered sequentially and
then re-enciphered. The cipher was broken, in fact broken twice as it
was changed a few days after it was first solved, and a general
procedure for interpreting the messages determined. Enciphering
was by a simple table, substituting one letter with another throughout
all the messages. Rotter started work in mid October but was kept
apart from the other code breakers until November, after he had broken
The intercepted messages were found to be intelligence reports on the
whereabouts of allied ships. This was interesting but not vital.
Russel Clarke now observed that similar coded messages were being
transmitted on short-wave, but were not being intercepted because of
shortages of receiving equipment, in particular the aerial. Hunstanton
was directed to stop listening to the military signals it had been
intercepting and instead monitor short-wave for a test period of one
weekend. The result was information about the movements of the High
Seas Fleet and valuable naval intelligence.
Hunstanton was permanently
switched to the naval signals and as a result stopped receiving
messages valuable to the military. Navy men who had been helping the
military were withdrawn to work on the naval messages, without
explanation, because the new code was kept entirely secret. The result
was a bad feeling between the naval and military interception services
and a cessation of cooperation between them, which continued into
The SKM (sometimes abbreviated SB in German documents) was the code
normally used during important actions by the German fleet. It was
derived from the ordinary fleet signal books used by both British and
German navies, which had thousands of predetermined instructions which
could be represented by simple combinations of signal flags or lamp
flashes for transmission between ships. The SKM had 34,300
instructions, each represented by a different group of three letters.
A number of these reflected old-fashioned naval operations, and did
not mention modern inventions such as aircraft. The signals used four
symbols not present in ordinary Morse code (given the names alpha,
beta, gamma and rho), which caused some confusion until all those
involved in interception learnt to recognise them and use a
standardised way to write them. Ships were identified by a
three-letter group beginning with a beta symbol. Messages not covered
by the predetermined list could be spelled out using a substitution
table for individual letters.
The sheer size of the book was one reason it could not easily be
changed, and the code continued in use until summer 1916. Even then,
ships at first refused to use the new codebook because the replacement
was too complicated, so the Flottenfunkspruchbuch (FFB) did not fully
replace the SKB until May 1917. Doubts about the security of the SKB
were initially raised by Behring, who reported that it was not
definitely known whether Magdeburg's code books had been destroyed or
not, and it was suggested at the court martial enquiry into the loss
that books might anyway have been recovered by Russians from the clear
shallow waters where the ship had grounded. Prince Heinrich of
Prussia, commander in chief of Baltic operations, wrote to the C-in-C
of the High Seas Fleet, that in his view it was a certainty that
secret charts had fallen into the hands of the Russians, and a
probability that the codebook and key had also. The German navy relied
upon the re-enciphering process to ensure security, but the key used
for this was not changed until 20 October and then not changed again
for another three months. The actual substitution table used for
enciphering was produced by a mechanical device with slides and
compartments for the letters. Orders to change the key were sent out
by wireless, and frequently confusion during the changeover period led
to messages being sent out using the new cipher and then being
repeated with the old. Key changes continued to occur infrequently,
only 6 times during 1915 from March to the end of the year, but then
more frequently from 1916.
There was no immediate capture of the FFB codebook to help the
Admiralty understand it, but instead a careful study was made of new
and old messages, particularly from the Baltic, which allowed a new
book to be reconstructed. Now that the system was understood, Room 40
reckoned to crack a new key within three to four days, and to have
reproduced the majority of a new codebook within two months. A German
intelligence report on the matter was prepared in 1934 by
Korvettenkapitän Kleikamp which concluded that the loss of
Magdeburg's codebook had been disastrous, not least because no steps
were taken after the loss to introduce new secure codes.
Capture of the HVB codebook
The second important code used by the German navy was captured at the
very start of the war in Australia, although it did not reach the
Admiralty until the end of October. The German-Australian steamer
Hobart was seized off
Port Phillip Heads near Melbourne on 11 August
1914. Hobart had not received news that war had broken out, and
Captain J. T. Richardson and party claimed to be a quarantine
inspection team. Hobart's crew were allowed to go about the ship but
the captain was closely observed, until in the middle of the night he
attempted to dispose of hidden papers. The Handelsverkehrsbuch (HVB)
codebook which was captured contained the code used by the German navy
to communicate with its merchant ships and also within the High Seas
Fleet. News of the capture was not passed to London until 9 September.
A copy of the book was made and sent by the fastest available steamer,
arriving at the end of October.
The HVB was originally issued in 1913 to all warships with wireless,
to naval commands and coastal stations. It was also given to the head
offices of eighteen German steamship companies to issue to their own
ships with wireless. The code used 450,000 possible four-letter groups
which allowed alternative representations of the same meaning, plus an
alternate ten-letter grouping for use in cables. Re-ciphering was
again used but for general purposes was more straightforward, although
changed more frequently. The code was used particularly by light
forces such as patrol boats, and for routine matters such as leaving
and entering harbour. The code was used by U-boats, but with a more
complex key. However, the complications of their being at sea for long
periods meant that codes changed while they were away and often
messages had to be repeated using the old key, giving immediate
information about the new one. German intelligence were aware in
November 1914 that the HVB code had fallen into enemy hands, as
evidenced by wireless messages sent out warning that the code was
compromised, but it was not replaced until 1916.
The HVB was replaced in 1916 by the Allgemeinefunkspruchbuch (AFB)
together with a new method of keying. The British obtained a good
understanding of the new keying from test signals, before it was
introduced for real messages. The new code was issued to even more
organisations than the previous one, including those in Turkey,
Bulgaria and Russia. It had more groups than its predecessor but now
of only two letters. The first copy to be captured came from a
Zeppelin but others were recovered from sunk U-boats.
Capture of the VB codebook
A third codebook was recovered following the sinking of German
SMS S119 in the Battle off Texel. In the middle of October
Battle of the Yser
Battle of the Yser was fought for control of the coastal
towns of Dixmude and Dunkirk. The British navy took part by bombarding
German positions from the sea and German destroyers were ordered to
attack the British ships. On 17 October Captain Cecil Fox commanding
the light cruiser Undaunted together with four destroyers,
HMS Lance, Lennox, Legion and Loyal, was ordered to intercept an
anticipated German attack and met four German destroyers (S115, S117,
S118, and S119) heading south from Texel with instructions to lay
mines. The German ships were outclassed and all were sunk after a
brief battle, whereupon the commander of S119 threw overboard all
secret papers in a lead-lined chest. The matter was dismissed by both
sides, believing the papers had been destroyed along with the ships.
However, on 30 November a British trawler dragged up the chest which
was passed to
Room 40 (Hall later claimed the vessel had been
searching deliberately). It contained a copy of the Verkehrsbuch (VB)
codebook, normally used by flag officers of the German Navy.
Thereafter the event was referred to by
Room 40 as "the miraculous
draft of fishes".
The code consisted of 100,000 groups of 5-digit numbers, each with a
particular meaning. It had been intended for use in cables sent
overseas to warships and naval attachés, embassies and consulates. It
was used by senior naval officers with an alternative Lambda key, none
of which failed to explain its presence on a small destroyer at the
start of the war. Its greatest importance during the war was that it
allowed access to communications between naval attachés in Berlin,
Madrid, Washington, Buenos Aires, Peking, and Constantinople.
In 1917 naval officers switched to a new code with a new key Nordo for
which only 70 messages were intercepted, but the code was also broken.
For other purposes VB continued in use throughout the war.
Re-ciphering of the code was accomplished using a key made up of a
codeword transmitted as part of the message and its date written in
German. These were written down in order and then the letters in this
key were each numbered according to their order of appearance in the
alphabet. This now produced a set of numbered columns in an apparently
random order. The coded message would be written out below these boxes
starting top left and continuing down the page once a row was filled.
The final message was produced by taking the column numbered '1' and
reading off its contents downward, then adding on the second column's
digits, and so on. In 1918 the key was changed by using the keywords
in a different order. This new cipher was broken within a few days by
Professor Walter Horace Bruford, who had started working for Room 40
in 1917 and specialised in VB messages. Two messages were received of
identical length, one in the new system and one in the old, allowing
the changes to be compared.
In early November 1914 Captain William Hall, son of the first head of
Naval Intelligence, was appointed as the new DID to replace Oliver,
who had first been transferred to Naval Secretary to the First Lord
and then Chief of the
Admiralty War Staff. Hall had formerly been
captain of the battlecruiser Queen Mary but had been forced to give up
sea duties due to ill health. Hall was to prove an extremely
successful DID, despite the accidental nature of his appointment.
Once the new organisation began to develop and show results it became
necessary to place it on a more formal basis than squatting in Ewing's
office. On 6 November 1914 the organisation moved to
Room 40 in the
Admiralty Old Building, which was by default to give it its name. Room
40 has since been renumbered, but still exists in the original
Admiralty Building off Whitehall, London, on the first floor, with
windows looking inwards to a courtyard wholly enclosed by Admiralty
buildings. Previous occupants of the room had complained that no one
was ever able to find it, but it was on the same corridor as the
Admiralty boardroom and the office of the First Sea Lord, Sir John
Fisher, who was one of the few people allowed to know of its
existence. Adjacent was the First Lord's residence (then Winston
Churchill), who was another of those people. Others permitted to know
of the existence of a signals interception unit were the Second Sea
Lord, the Secretary of the Admiralty, the Chief of Staff (Oliver), the
Director of Operations Division (DOD) and the assistant director, the
Director of Intelligence Division (DID, Captain William Hall) and
three duty captains. Admiral Sir Arthur Wilson, a retired First Sea
Lord, had returned to the admiralty to work with the staff and was
also included in the secret. The Prime Minister may also have been
All messages received and decoded were to be kept completely secret,
with copies only being passed to the Chief of Staff and Director of
Intelligence. It was decided that someone from the intelligence
department needed to be appointed to review all the messages and
interpret them from the perspective of other information. Rotter was
initially suggested for the job, but it was felt preferable to retain
him in code breaking and Commander Herbert Hope was chosen, who had
previously been working on plotting the movements of enemy ships. Hope
was initially placed in a small office in the west wing of the
Admiralty in the intelligence section, and waited patiently for the
few messages which were approved for him to see. Hope reports that he
attempted to make sense of what he was given and make useful
observations about them, but without access to the wider information
being received his early remarks were generally unhelpful. He reported
to Hall that he needed more information, but Hall was unable to help.
On 16 November, after a chance meeting with Fisher where he explained
his difficulties, Hope was granted full access to the information
together with instructions to make twice daily reports to the First
Sea Lord. Hope knew nothing of cryptanalysis or German, but working
with the code breakers and translators he brought detailed knowledge
of naval procedures to the process, enabling better translations and
then interpretations of received messages. In the interests of secrecy
the intention to give a separate copy of messages to the DID was
dispensed with so that only the Chief of Staff received one, and he
was to show it to the
First Sea Lord
First Sea Lord and Arthur Wilson.
As the number of intercepted messages increased, it became part of
Hope's duties to decide which were unimportant and should just be
logged, and which should be passed on outside Room 40. The German
fleet was in the habit each day of reporting by wireless the position
of each ship, and giving regular position reports when at sea. It was
possible to build up a precise picture of the normal operation of the
High Seas Fleet, indeed to infer from the routes they chose where
defensive minefields had been placed and where it was safe for ships
to operate. Whenever a change to the normal pattern was seen, it
signalled that some operation was about to take place and a warning
could be given. Detailed information about submarine movements was
available. Most of this information, however, was retained wholly
Room 40 although a few senior members of the
kept informed, as a huge priority was placed by the Staff upon keeping
secret the British ability to read German transmissions.
Jellicoe, commanding the Grand Fleet, on three occasions requested
Admiralty that he should have copies of the codebook which
his cruiser had brought back to Britain, so that he could make use of
it intercepting German signals. Although he was aware that
interception was taking place, little of the information ever got back
to him, or it did so very slowly. No messages based upon Room 40
information were sent out except those approved by Oliver personally
(except for a few authorised by the First Lord or First Sea Lord).
Although it might have been impractical and unwise for code breaking
to have taken place on board ship, members of
Room 40 were of the view
that full use was not being made of the information they had
collected, because of the extreme secrecy and being forbidden to
exchange information with the other intelligence departments or those
Signals interception and direction finding
The British and German interception services began to experiment with
direction-finding radio equipment in the start of 1915. Captain Round,
working for Marconi, had been carrying out experiments for the army in
France and Hall instructed him to build a direction-finding system for
the navy. At first this was sited at Chelmsford but the location
proved a mistake and the equipment was moved to Lowestoft. Other
stations were built at Lerwick, Aberdeen, York, Flamborough Head and
Birchington and by May 1915 the
Admiralty was able to track German
submarines crossing the North Sea. Some of these stations also acted
as 'Y' stations to collect German messages, but a new section was
Room 40 to plot the positions of ships from the
directional reports. A separate set of five stations was created in
Ireland under the command of the Vice Admiral at Queenstown for
plotting ships in the seas to the west of Britain and further stations
both within Britain and overseas were operated by the Admiral
The German navy knew of British direction-finding radio and in part
this acted as a cover, when information about German ship positions
was released for operational use. The two sources of information, from
directional fixes and from German reports of their positions,
complemented each other.
Room 40 was able to observe, using
intercepted wireless traffic from Zeppelins which were given position
fixes by German directional stations to help their navigation, that
the accuracy of British systems was better than their German
counterparts. This was explainable by the wider baseline used in
Room 40 had very accurate information on the positions of German ships
but the Admiralty's priority remained to keep the existence of this
knowledge secret. Hope was shown the regular reports created by the
Intelligence Division about German ship whereabouts so that he might
correct them. This practice was shortly discontinued, for fear of
giving away their knowledge. From June 1915, the regular intelligence
reports of ship positions were no longer passed to all flag officers,
only to Jellicoe, who was the only person to receive accurate charts
of German minefields prepared from
Room 40 information. Some
information was passed to Beatty (commanding the battlecruisers),
Tyrwhitt (Harwich destroyers) and Keyes (submarines) but Jellicoe was
unhappy with the arrangement. He requested that Beatty should be
issued with the Cypher B (reserved for secret messages between the
Admiralty and him) to communicate more freely and complained that he
was not getting sufficient information.
Zimmermann telegram as decoded by Room 40
All British ships were under instructions to use radio as sparingly as
possible and to use the lowest practical transmission power. Room 40
had benefited greatly from the free chatter between German ships,
which gave them many routine messages to compare and analyse, and from
the German habit of transmitting at full power, making the messages
easier to receive. Messages to Scapa were never to be sent by
wireless, and when the fleet was at sea, messages might be sent using
lower power and relay ships (including private vessels), to make
German interception more difficult. No attempts were made by the
German fleet to restrict its use of wireless until 1917 and then only
in response to perceived British use of direction finding, not because
it believed messages were being decoded.
Room 40 played an important role in several naval engagements during
the war, notably in detecting major German sorties into the North Sea
that led to the Battle of Dogger Bank in 1915 and the Battle of
Jutland in 1916, as the British fleet was sent out to intercept them.
Its most notable contribution was in decrypting the Zimmermann
Telegram, a telegram from the German Foreign Office sent in January
1917 via Washington to its ambassador
Heinrich von Eckardt in
Mexico. This interception had been made possible a few hours after
Britain entered the war by the cable ship Alert (though often
incorrectly attributed to Telconia), which stood off the German coast
and cut the five telegraph cables connecting Germany with Spain,
Tenerife and New York. The decryption was described as the most
significant intelligence triumph for Britain during World War I,
and one of the earliest occasions on which a piece of signals
intelligence influenced world events.
In the telegram's plaintext,
Nigel de Grey and William Montgomery
learned of German Foreign Minister Arthur Zimmermann's offer to Mexico
of United States' territories of Arizona, New Mexico, and Texas as an
enticement to join the war as a German ally. The telegram was passed
to the U.S. by Captain Hall, and a scheme was devised (involving a
still unknown agent in
Mexico and a burglary) to conceal how its
plaintext had become available and also how the U.S. had gained
possession of a copy. The telegram was made public by the United
States, which declared war on Germany on 6 April 1917, entering the
war on the Allied side.
Other staff of
Room 40 included Frank Adcock, John Beazley,
Francis Birch, Walter Horace Bruford, William 'Nobby' Clarke, Alastair
Frank Cyril Tiarks and Dilly Knox.
Merger with Military Intelligence (MI)
Room 40 was deactivated and its function merged with the
British Army's intelligence unit
MI1b to form the Government Code and
Cypher School (GC&CS). This unit was housed at Bletchley Park
during the Second World War and subsequently renamed Government
Communications Headquarters (GCHQ) and relocated to Cheltenham.
^ a b Massie 2004, pp. 314–317.
^ Tuchman 1958, pp. 20-21.
^ Massie 2004, p. 580.
^ Lieutenant Commander James T. Westwood, USN. "Electronic Warfare and
Signals Intelligence at the Outset of World War I" (PDF). NSA.
Retrieved 2009-05-04. After the war, it was estimated that
Room 40 had
solved some 15,000 German naval and diplomatic communications, a very
great number considering that recoveries were hand-generated.
^ Johnson 1997, pp. 32.
^ Winkler 2009, pp. 848–849.
^ Beesly 1982, pp. 2, 8–9.
^ a b Beesly 1982, pp. 11–12.
^ Andrew 1986, p. 87.
^ Beesly 1982, pp. 12–14.
^ Beesly 1982, pp. 4–5.
^ Beesly 1982, pp. 5–6.
^ Beesly 1982, pp. 14–15.
^ Andrew 1986, p. 90.
^ Beesly 1982, p. 15.
^ Denniston 2007, p. 32.
^ Beesly 1982, pp. 22–23.
^ Beesly 1982, pp. 23–26.
^ Beesly 1982, p. 25.
^ Beesly 1982, pp. 3–4.
^ Beesly 1982, p. 26.
^ Beesly 1982, pp. 26–27.
^ Beesly 1982, pp. 6–7.
^ Beesly 1982, pp. 27, 28.
^ Beesly 1982, pp. 27–28.
^ Beesly 1982, pp. 15–19.
^ Beesly 1982, pp. 18–20.
^ a b Beesly 1982, pp. 40–42.
^ Beesly 1982, pp. 69–70.
^ Beesly 1982, p. 70.
^ a b Beesly 1982, pp. 70–72.
^ a b "Why was the Zimmerman
Telegram so important?". BBC. 17 January
2017. Retrieved 17 January 2017. It was, many believed, the single
greatest intelligence triumph for Britain in World War One.
^ a b "The telegram that brought America into the First World War".
BBC History Magazine. 17 January 2017. Retrieved 17 January
^ Martin Robertson; David Gill (2004). "Beazley, Sir John Davidson
(1885–1970)". Oxford Dictionary of National Biography. Oxford
University Press. Retrieved 15 June 2012.
^ Erskine & Smith 2011, p. 14
Andrew, Christopher (1986). Her Majesty's Secret Service: The Making
of the British Intelligence Community. New York: Viking.
Beesly, Patrick (1982). Room 40: British Naval Intelligence,
1914–1918. London: Hamish Hamilton. ISBN 0-241-10864-0.
Denniston, Robin (2007). Thirty secret years: A.G. Denniston's work
for signals intelligence 1914–1944. Polperro Heritage Press.
Erskine, Ralph; Smith, Michael, eds. (2011). The Bletchley Park
Codebreakers. Updated and extended version of Action This Day: From
Breaking of the Enigma Code to the Birth of the Modern Computer.
Bantam Press, 2001. Biteback. ISBN 978-1-84954-078-0.
Gannon, Paul, (2011). Inside Room 40: The Codebreakers of World War I.
Ian Allen Publishing, London, ISBN 978-0-7110-3408-2
Hoy, Hugh Cleland, (1932) 40 O.B. or How the War Was Won. Hutchison
& Co. London,
Johnson, John (1997). The Evolution of British Sigint, 1653–1939.
London: HMSO. OCLC 52130886.
Massie, Robert K. (2004). Castles of Steel: Britain, Germany and the
Winning of the Great War at Sea. London: Jonathan Cape.
Room 40 Compromise, ms., U.S. National Security Agency, 19600101
1960 Doc 3978516
Tuchman, Barbara (1958). The Zimmermann Telegram. New York: Ballantine
Books. ISBN 0-345-32425-0.
Winkler, Jonathan Reed (July 2009). "Information Warfare in World War
I". The Journal of Military History. 73: 845–867.
doi:10.1353/jmh.0.0324. ISSN 1543-7795.
James Wyllie; Michael McKinley (2016). The Codebreakers: The Secret
Intelligence Unit That Changed the Course of the First World War.
Ebury Publishing. ISBN 978-0-09-195773-5.
The Papers of William Clarke, who worked in Room 40, are held at
Churchill Archives Centre
Churchill Archives Centre in Cambridge and are accessible to the
The Papers of Alexander Denniston, second in command of Room 40, are
also held at Churchill Archives Centre.
The Papers of William Reginald Hall, joint founder of Room 40, are
also held at Churchill Archives Centre.
Original Documents from Room 40: LUSITANIA case; Naval Battle of
Jutland/Skagerrak; The Zimmermann/
Mexico Telegram; German Submarine
Room 40 Intelligence in general; PhotoCopies from The
National Archives, Kew, Richmond, UK.
World War I
Sinai and Palestine
Asian and Pacific
German New Guinea and Samoa
North Atlantic U-boat campaign
Indian, Pacific and South Atlantic Oceans
Más a Tierra
Scramble for Africa
Scramble for Africa (1880–1914)
Russo-Japanese War (1905)
First Moroccan (Tangier) Crisis (1905–06)
Agadir Crisis (1911)
Italo-Turkish War (1911–12)
French conquest of Morocco
French conquest of Morocco (1911–12)
First Balkan War
First Balkan War (1912–13)
Second Balkan War
Second Balkan War (1913)
Anti-Serb riots in Sarajevo
Battle of the Frontiers
Battle of Cer
First Battle of the Marne
Siege of Tsingtao
Battle of Tannenberg
Battle of Galicia
Battle of the Masurian Lakes
Battle of Kolubara
Battle of Sarikamish
Race to the Sea
First Battle of Ypres
Second Battle of the Masurian Lakes
Second Battle of Ypres
Battle of Gallipoli
Second Battle of Artois
Battles of the Isonzo
Second Battle of Champagne
Siege of Kut
Battle of Loos
Battle of Verdun
Lake Naroch Offensive
Battle of Asiago
Battle of Jutland
Battle of the Somme
Battle of Romani
Battle of Transylvania
Capture of Baghdad
First Battle of Gaza
Second Battle of Arras
Second Battle of the Aisne
Third Battle of Ypres (Passchendaele)
Battle of Mărășești
Battle of Caporetto
Southern Palestine Offensive
Battle of Cambrai
Armistice of Erzincan
Treaty of Brest-Litovsk
Second Battle of the Marne
Battle of Baku
Hundred Days Offensive
Battle of Megiddo
Third Transjordan attack
Battle of Vittorio Veneto
Battle of Aleppo
Armistice of Salonica
Armistice of Mudros
Armistice of Villa Giusti
Armistice with Germany
Mexican Revolution (1910–20)
Somaliland Campaign (1910–20)
Libyan resistance movement (1911–43)
Maritz Rebellion (1914–15)
Zaian War (1914–21)
Indo-German Conspiracy (1914–19)
Senussi Campaign (1915–16)
Volta-Bani War (1915–17)
Easter Rising (1916)
Anglo-Egyptian Darfur Expedition
Anglo-Egyptian Darfur Expedition (1916)
Kaocen Revolt (1916–17)
Central Asian Revolt (1916-17)
Russian Revolution (1917)
Finnish Civil War
Finnish Civil War (1918)
Russian Civil War
Russian Civil War (1917–21)
Ukrainian–Soviet War (1917–21)
Armenian–Azerbaijani War (1918–20)
Georgian–Armenian War (1918)
German Revolution (1918–19)
Revolutions and interventions in Hungary (1918–20)
Hungarian–Romanian War (1918–19)
Greater Poland Uprising (1918–19)
Estonian War of Independence
Estonian War of Independence (1918–20)
Latvian War of Independence
Latvian War of Independence (1918–20)
Lithuanian Wars of Independence
Lithuanian Wars of Independence (1918–20)
Third Anglo-Afghan War
Third Anglo-Afghan War (1919)
Egyptian Revolution (1919)
Polish–Ukrainian War (1918–19)
Polish–Soviet War (1919–21)
Irish War of Independence
Irish War of Independence (1919–21)
Turkish War of Independence
Greco-Turkish War (1919–22)
Turkish–Armenian War (1920)
Iraqi revolt (1920)
Polish–Lithuanian War (1920)
Vlora War (1920)
Franco-Syrian War (1920)
Soviet–Georgian War (1921)
Irish Civil War
Irish Civil War (1922–23)
Schlieffen Plan (German)
Plan XVII (French)
Last surviving veterans
1918 flu pandemic
Destruction of Kalisz
Rape of Belgium
German occupation of Belgium
German occupation of Luxembourg
German occupation of northeastern France
Pontic Greek genocide
Blockade of Germany
German prisoners of war in the United States
Partition of the Ottoman Empire
Agreement of Saint-Jean-de-Maurienne
Paris Peace Conference
Treaty of Brest-Litovsk
Treaty of Lausanne
Treaty of London
Treaty of Neuilly
Treaty of St. Germain
Treaty of Sèvres
Treaty of Trianon
Treaty of Versailles
League of Nations
World War I
World War I memorials
United Kingdom intelligence agencies
Security Service (MI5)
Centre for the Protection of National Infrastructure
National Crime Agency
National Crime Agency (NCA)
National Ballistics Intelligence Service
National Ballistics Intelligence Service (NBIS)
National Fraud Intelligence Bureau
Metropolitan Police Service
Metropolitan Police Service Specialist Operations
Counter Terrorism Command
National Domestic Extremism and Disorder Intelligence Unit
National Counter Terrorism Policing Network
Secret Intelligence Service
Secret Intelligence Service (SIS/MI6)
Defence Intelligence (DI)
Defence Intelligence Fusion Centre
Joint Intelligence Training Group
Joint Support Group
1st Intelligence, Surveillance and Reconnaissance Brigade
30 Commando Information Exploitation Group
Government Communications Headquarters
Government Communications Headquarters (GCHQ)
National Cyber Security Centre
Joint Intelligence Organisation (JIO)
Joint Terrorism Analysis Centre (JTAC)
Joint Intelligence Committee (JIC)
National Security Council
National Security Adviser
Office for Security and Counter-Terrorism
Foreign and Commonwealth Office
Ministry of Defence
Chiefs of Staff Committee
Joint Forces Command
Single Intelligence Account
National Security Strategy
Intelligence and Security Committee of Parliament
Independent Reviewer of Terrorism Legislation
Investigatory Powers Tribunal
Intelligence Services Commissioner
Interception of Communications Commissioner
Directorate of Military Intelligence
Naval Intelligence Department
Naval Intelligence Division
No. 30 Commando
Special Operations Executive (SOE)
Far East Combined Bureau
Force Research Unit
Special Reconnaissance Unit