First, the signing of the German–Soviet non-aggression agreement on 23 August 1939 prior to the invasion of Poland to secure the new eastern border with the Soviet Union, prevent the emergence of a two-front war, and to circumvent a shortage of raw materials due to an expected British naval blockade. Second, the Blitzkrieg attacks in northern and western Europe (Operation Weserübung and the Battle of France respectively) to neutralize opposition from the west. This resulted in the conquest of Denmark, Norway, Luxembourg, Belgium, the Netherlands, and France, all of which were under German rule by the early summer of 1940.