the sinkings in Norwegian territorial waters of several British ships by German U-boats. In the following months, aircraft from all the belligerents violated Norwegian neutrality. Almost immediately after the outbreak of war, the British began pressuring the Norwegian government to provide them with the services of the Norwegian merchant navy, being in dire need of shipping to oppose the strength of Nazi forces. Following protracted negotiations between 25 September and 20 November 1939, the Norwegians agreed to charter 150 tankers, as well as other ships with a tonnage of 450,000 gross tons. The Norwegian government's concern for the country's supply lines played an important role in persuading them to accept the agreement. Value of Norway Norway, although neutral, was considered strategically important for both sides for several reasons. First was the importance of the iron ore from Sweden – upon which Germany depended – that was exported through the Norwegian port of Narvik. This route was especially important during the winter months when much of the Baltic Sea was frozen over. Narvik became of greater significance to the British when it became apparent that Operation Catherine, a plan to gain control of the Baltic Sea, would not be realized. Großadmiral Erich Raeder had pointed out several times in 1939 the danger to Germany of Britain seizing the initiative and launching its own invasion in Scandinavia, for if the powerful Royal Navy had bases at Bergen, Narvik and Trondheim, the North Sea would be virtually closed to Germany, and the Kriegsmarine would be at risk even in the Baltic. Controlling Norway would also be a strategic asset in the Battle of the Atlantic. The capture of ports would create gaps in the blockade of Germany, giving access to the Atlantic Ocean. These ports would allow Germany to use its sea power effectively against the Allies. Control of Norwegian air bases would allow German reconnaissance aircraft to operate far into the North Atlantic, while German U-boats and surface ships operating out of Norwegian naval bases would be able to break the British blockade line across the North Sea and attack convoys heading to Great Britain. Winter War When the Soviet Union launched its attack against Finland on 30 November 1939, the Allies found themselves aligned with Norway and Sweden in support of Finland against the much larger aggressor. After the outbreak of the Winter War between Finland and the Soviet Union, Norway