In early 1940, the Allies approved a naval campaign designed to seize the northern part of Norway, a neutral country, including the key port of Narvik, and possibly also to seize the iron mines at Gällivare in northern Sweden, from which Germany obtained much of its iron ore. The British planned to invade Norway in March 1940 to use the ports as bases for the Royal Navy in order to encircle Germany, but the operation (codenamed Plan R 4) was postponed. The British and French had previously planned to invade Sweden, in order to assist Finland during the Winter War against the Soviet Union.
Adolf Hitler found out about Plan R 4, and militarily intervened as the British began Operation Wilfred by laying mines in neutral Norwegian waters.
