Swedish iron ore was an important economic factor in the European Theatre of World War II. Both the Allies and the Third Reich were keen on the control of the mining district in northernmost Sweden, surrounding the mining towns of Gällivare and Kiruna. The importance of this issue increased after other sources were cut off from Germany by the British sea blockade during the Battle of the Atlantic. Both the planned Anglo-French support of Finland in the Winter War, and the following German occupation of Denmark and Norway (Operation Weserübung) were to a large extent motivated by the wish to deny their respective enemies iron critical for wartime production of steel.