The interpretation of the aerial photos, every bit as important as taking them, lagged even further behind, and it was not until July 1940, after pressure from the Admiralty, that the Air Ministry set up the Photographic Interpretation Unit (PIU) to interpret photographs for all three services and to start harvesting its real operational value.35
On the German side, things were far more advanced. For over ten years after WWI, the radio monitoring service or B-Dienst of the Kaiserliche Marine lived its own life, monitoring technological development and code breaking until October 1929, when it once again became part of the navy and was moved to Kiel. Much of its work was devoted to the creation of new cyphers and codes and it was in this period that the Enigma machine emerged. In the late 1930s, the B-Dienst was brought to Berlin as part of the Office of Communications and Intelligence. The staffs were increased and also tasked with collecting naval information from foreign press, public sources and German connections around the world.36
Two chains of coastal radio stations were developed during the early 1930s: one for the North Sea on the Frisian Islands, equipped with receivers, transmitters and radio direction finders and one for the Baltic along the German east coast. Bearings obtained on transmissions by two or more radio stations could be triangulated to obtain a fairly accurate position depending on the strength and quality of the signal. By 1939, the stations were equipped with state-of-the art receivers and radio direction finders as well as communication tools for interception, location and intelligence analysis, far superior to anything available to the Allies.37
The B-Dienst and its decryption service, the xB-Dienst, could at the outbreak of the war read and decypher a significant amount of encrypted Allied radio communications, including the most widely used code of the Royal Navy. The B-Dienst operators could on a routine basis monitor the positions of all British warships; their recognition codes and a large part of the signals they sent between themselves and between the Admiralty and the various C-in-Cs were understandable. Decrypting and dispersal of the information took time, though, and could not always be acted on in time to make a difference. It is clear from the war diary of the SKL and BdU (Befehlshaber der Unterseeboote – C-in-C Submarines) that even if the signals themselves could not be read in full, the B-Dienst could on an almost continuous basis monitor the whereabouts of most British and French capital ships and cruisers in the North Sea, Atlantic and Mediterranean. Signals to the British merchant navy were to a large extent transmitted in plain language long after the outbreak of the war, and it was not until January 1940 that the British introduced the Merchant Navy Code.38
German military intelligence in the first phase of the war was handled by the OKW/Amts-gruppe Auslandsnachrichten (Bureau of Foreign Intelligence), usually known as the Abwehr. It was led by Admiral Wilhelm Canaris, a firm German patriot but no friend of Hitler and his Nazi Party. Based on a counter-espionage organisation founded in the 1920s, the Abwehr had lived a clandestine life under various guises until it was brought directly under the OKW in 1938, giving Canaris unprecedented influence. Within the Abwehr, Group I handled military and economic intelligence, Group II sabotage and Group III counter-intelligence. Every morning at 10:00, Admiral Canaris and his Chief of Staff Colonel Oster met with the group leaders to hear their reports and give instructions. Thus the leaders of the Abwehr were well informed of what went on in the bureau as well as in the OKW and the Foreign Office.39
— 6 —
Operation Catherine
IN CHURCHILL’S MIND, A naval expedition into the Baltic, in alliance with the Soviet Union, would open the door to Germany’s back, diverting forces from the Western Front and severing the iron ore supplies from Sweden. By the time he returned to the Admiralty, though, Germany was about to secure the southern Baltic coastline with the Soviet Union as an ally. Nevertheless, Admiral of the Fleet Lord Cork and Orrery, an old friend of Churchill, was given the task of preparing Operation Catherine, a ‘forced passage into the Baltic’.1 A substantial naval force was foreseen: three Revenge-class battleships, an aircraft carrier, five cruisers, two destroyer flotillas, submarines and supporting auxiliaries. Once inside the Baltic, the use of Swedish bases was taken for granted. Pound and most of the Admiralty expressed strong doubts from the start. The resources were needed elsewhere and the ships sent into the Baltic would in all likelihood be lost, weakening the Royal Navy at a time when this could not be afforded. During the winter, Churchill gradually lost his enthusiasm for the plan as he realised that he could rely on neither Swedish bases nor Russian cooperation. Instead, he decided that a strong force of submarines should be prepared to enter the Baltic should an opportunity arise. All plans were eventually cancelled when Finland capitulated in March 1940.2
Extensive Mine-laying
On the conclusion of the Polish campaign, three main tasks were defined by the SKL for Naval Group East: securing the Kattegat and the entrances to the Baltic against Polish submarines that might still be inside and British ones that might attempt to enter, controlling commercial shipping to prevent contraband to and from Finland and the Baltic, and securing the sea lanes between Germany and East Prussia. Admiral Carls took charge in the east on 31 October when Generaladmiral Albrecht retired. Vizeadmiral Hermann Mootz was responsible for the minefields and other defensive measures as C-in-C Security, Baltic (Befehlshaber der Sicherung der Ostsee – BSO). After the Polish campaign, the minesweeper forces were split into an eastern command under Kapitän Ruge in Swinemünde and a western command under Konteradmiral Hans Stohwasser in Cuxhaven, reporting to the respective eastern and western Naval Group Commands.3
The first German minefields in the Baltic were established in the early morning of 4 September. A total of 564 mines were laid by Hansestadt Danzig and Tannenberg south of Trelleborg in Sweden, closing off the northern part of the exit from Øresund. At the same time, Brummer, supported by M75, M84 and M85, laid a similar but smaller field closer to Denmark. Tannenberg augmented the fields the next day, closing them towards Danish and Swedish territorial waters. These fields were known as Undine I, II and III respectively, and were later further enhanced by A/S mines. South of the Great Belt, Preussen, fresh off the yard after conversion to a minelayer, laid 136 mines on the 4th in the field Grosser Bär off the Danish island of Langeland.
Admiral Rolf Carls (left) in charge of Marinegruppenkommando Ost – Naval Group Command East. With him is Korvettenkapitän Karl-Friedrich Brill of minelayer Cobra. (Author’s collection)
These mining operations were extremely efficient in spite of most men and ships being new to the trade. Undine I and II were laid in 90 minutes, the two rows of Grosser Bär in 80 minutes. In the following days, the latter field was augmented by the stripes Jade-5 and Jade-6 off Lolland, two A/S minefileds south (Jade-3 and Jade-4) as well as two A/S nets off Falster (see map opposite).
The German minelayers had their ordnance lined up on two tracks, one on each side, going over the stern aft. This made it easy for the pilot and the captain to focus on keeping course and speed while the mining officer ensured the mines were dropped at regular intervals, even in a heavy swell. An unexpected challenge was that the maps had inaccurate depth annotations, and each mine-laying operation had to be preceded by a calibration survey to ensure the anchors were pre-set correctly so the mines ended up at the right depth.
The 3,933-ton Greek steamer Kosti, en route from Leningrad to Antwerp with props, ventured into the Grosser Bär Field in the morning of the 4th, before appropriate warning had been set up, triggering a mine. The load of props kept the freighter floating for a while and the crew took to the boats, eventually landing near Malmø. Late in the evening of the same day, the small German 250-ton motor schooner Lianne also entered one of the minefields, sinking with all five of her crew being lost. To prevent further accidents, pilot boats were stationed on each