Meanwhile, another offensive was being prepared. While the land war in the west was in suspense, the sea war against England was active. In his Memorandum of 9th October, Hitler had declared that, used ruthlessly, the submarine can today be an extraordinary threat to Britain; but he added that Germany's submarine routes were long and exposed. The sole remaining approach and departure route for our submarines lay between Norway and the Shetland Islands. The British had laid an extensive mine barrage across this route in the war of 1914, and it was inconceivable, he wrote, that, in a long war, they would not do so again. Therefore it was essential to forestall or circumvent such barriers by submarine bases on the Atlantic coast. The destruction of the surface raiding pocket battleship Graf Spee after the Battle Of The River Plate of 13th December, 1939, emphasised Germany's dependence on submarines, which in turn relied on the use of Norwegian territorial waters. Moreover, Germany was also dependent on Swedish iron ore which, in winter, had to be brought from the North Norwegian port of Narvik through the same waters. Control of Norwegian territorial waters, direct or indirect, thus became a major interest of both Britain and Germany, and during the preparations for an offensive in the west, Hitler ordered plans to be made for the seizure and occupation of both Denmark and Norway, under the code name Fall Weserübung (Case Weser River Exercise). The first order was given on 14th December, 1939, immediately after the Battle Of The River Plate, following a meeting between Hitler and his Norwegian ally, Vidkun Quisling. On 1st March, 1940, Hitler issued the formal Directive -- which, not being concerned with the main theatre, was not numbered in the series. It is here numbered 10a.