The Phoney War was an eight-month period at the start of World War II, during which there was only one limited military land operation on the Western Front, when French troops invaded Germany's Saar district. Nazi Germany carried out the invasion of Poland on 1 September 1939; the Phoney period began with the declaration of war by the United Kingdom and France against Nazi Germany on 3 September 1939, after which little actual warfare occurred, and ended with the German invasion of France and the Low Countries on 10 May 1940. Although there was no large-scale military action by Britain and France, they did begin some economic warfare, especially with the naval blockade, and shut down German surface raiders. They created elaborate plans for numerous large-scale operations designed to cripple the German war effort. These included opening an Anglo-French front in the Balkans, invading Norway to seize control of Germany's main source of iron ore and a strike against the Soviet Union, to cut off its supply of oil to Germany. By April 1940, the lone execution of the Norway plan was considered inadequate to stop the German offensive.