German U - boat bases in occupied Norway German U - boat bases in occupied Norway existed between 1940 and 1945 , when the German Navy , the " Kriegsmarine " , converted several naval base s in Norway into Submarine base s . Norwegian coastal cities became available to the German Navy after the invasion of Denmark and Norway in April 1940 . Following the conclusion of the Norwegian Campaign , the occupying German forces began to transfer U - boats stationed in Germany to many Norwegian port cities such as Bergen , Narvik , Trondheim , Hammerfest and Kirkenes . Initial planning for many U - boat bunkers began in late 1940 and starting in 1941 , the Todt Organisation began the construction of bunkers in Bergen and Trondheim . These bunkers were completed between 1942 and 1943 . U - boats stationed in Norway were generally used to extend the German Navy ' s range of operation in the North Atlantic and Arctic Oceans . These naval bases were used to house U - boats that would take part in the interception of Allied convoys crossing the Arctic Ocean to the Soviet Union . It was not until the liberation of France by the Western Allies that U - boat activity in many Norwegian ports increased . With the loss of the French ports , many German U - boats were re - located to Norwegian port cities . During the extent of the German occupation of Norway , over 240 U - boats were stationed in the Nordic country at one time or another , most of which were members of the 11th U - boat Flotilla which had 190 U - boats in its fleet during the flotilla ' s entire career . Other well - known flotillas in Norway were the 13th and 14th Flotillas . German invasion of Norway . Germany invaded both Norway and Denmark on 9 April 1940 . Norway was very important to Germany for two reasons : firstly as a base for naval units to harass Allied shipping in the North Atlantic and Arctic Oceans , and secondly to secure shipments of iron ore coming in from Sweden through the port of Narvik . During Operation Weserübung , Denmark fell in less than a day , becoming the briefest invasion in recorded military history . The Norwegians put up a stiffer resistance to the invading German forces . Nonetheless , by May the southern half of Norway was under German control . Following the German invasion of France and the Low Countries , the Allies were forced to evacuate Narvik , leaving the country to the Germans who would occupy it until the end of the war . After the invasion , Vidkun Quisling led the collaborationist government of Norway , based around the fascist Nasjonal Samling ( " National Gathering " ) party . German use of Norwegian ports . During the occupation , several of the nation ' s naval ports were turned into U - boat bases that were used to harass Allied shipping in the North Atlantic and Arctic Oceans . These included Bergen , Narvik , Trondheim , Hammerfest and Kirkenes . Over 240 U - boats were stationed in Norway at various times during the war , most of them were members of the 11th U - boat Flotilla which had 190 U - boats in its fleet during the flotilla ' s career . Other well - known flotillas in Norway included the 13th and 14th flotilla s . Bergen . The southern port of Bergen was captured by the Germans on 9 April 1940 , on the first day of the invasion . The Germans immediately saw the potential for several Norwegian harbours and ports to function as bases of operation for the " Kriegsmarine " ' s U - boats patrolling the North Sea and the Arctic Ocean . Bergen was the first Norwegian port to be established as a U - boat base . From July 1940 to the end of the war in May 1945 , 270 U - boat patrols originated in Bergen . Bergen was also the site where the only type XXI submarine left for a patrol in the war ; U - 2511 left the port on 3 May 1945 . Planning for the first U - boat bunker in Bergen began in late 1940 and was undertaken by the German military engineering group , the Todt Organisation . It coordinated the building of the first U - boat bunker in Bergen , codenamed " Bruno " , as well as several other U - boat bunkers along the Norwegian coast . The construction of " Bruno " began in 1941 . When it was completed it had seven pens , three of which were " dry " docks and three of which were " wet " docks . The seventh pen was used for storing fuel , torpedoes and other essentials . The bunker itself had a roof up to thick and was by in area . Once " Bruno " was completed , it could hold up to 9 U - boats . Bergen remained a quiet base during the first few years of the occupation , experiencing only one large Allied air attack . In 1943 , the U - boat base added two new bunkers , " U - Stützpunkt Prien " and " U - Stützpunkt Weddingen " along with a shipyard named " Danziger Werft " . The U - boat base at Bergen grew exponentially after the liberation of France by the Western Allies , when the senior commanding officer of U - boat operations in the west ( " FdU West " ) was moved from Angers , France to the city . A large expansion program was conducted at the base and several new anti - aircraft guns were set up to protect the U - boat bunkers . More engineers and technicians were moved to Bergen to accommodate the increased technical requirements of the base as well . Two more bunkers named " Werft Gemeinschaftslager 1 " and " Gemeinschaftslager 2 " were also built during this time . In late 1944 several British air attacks damaged the U - boat bases in Bergen . During one of these attacks , a ' Tallboy ' bomb went straight through the roof of pen 3 , damaging it beyond repair and putting it out of action for the remainder of the war . The base was handed over to the Allies , along with the rest of occupied Norway , on 8 May 1945 when all German forces in Europe surrendered . As a result of the intensive bombing of the city in late 1944 and 1945 , the Bergen area was one of the worst damaged regions in the south of Norway at the conclusion of the war . Trondheim . Trondheim was an important U - boat base in Norway during the war . It was the home of the 13th flotilla and sent out around 55 U - boat patrols between June 1940 and the end of the war in May 1945 . Two U - boat bunkers , codenamed " Dora I " and " Dora II " , were planned to be built in Trondheim to provide additional repair facilities outside of Germany itself . Like the bunkers in Bergen , the two bunkers in Trondheim came under the control of the Todt Organisation . Only " Dora I " , which the " Kriegsmarine " took charge of in 1943 , was completed before the end of the war . At long and wide it had five pens with space to hold seven U - boats . Construction of " Dora II " began in 1942 but was never completed . If finished it would have been long by wide , with four pens capable of holding six U - boats .