The declaration of war by the Western powers on 3 September 1939 surprised Hitler. Thus, he found himself in a war with Britain and France that he had not wanted in 1939. In his ‘Directive No. 9’ of 29 November 1939, ‘Principles of warfare against the economy of the enemy’, he considered interference with the British economy as the ‘most effective means’ to defeat Britain. Hitler was prepared to adopt this strategic concept for the navy, hitting Britain where she was most vulnerable by disrupting her sea lines of communication.
However, Hitler and his Naval War Staff operated on different planes of strategic thinking: Hitler expected a short war limited to Europe and did not want to jeopardise the hope of better relations with Great Britain. The Naval War Staff, on the other hand, was convinced that the conflict with Britain would be long. It would have to be won in the Atlantic, even if that meant the entry of the United States into the war.
Because of the inaction of the Western Allies, Hitler even believed that they had only declared war to save face. After the end of the fighting in Poland, Hitler therefore made a ‘peace offer’. However, for Paris and London there could no longer be any question of negotiating with Hitler. The strategy of the Allies was guided by the wish to disrupt the concentration of German forces on the border of France. In the spring of 1940, the spread of the war to Scandinavia gave the Allies the desired secondary theatre, but no opportunity to thwart Hitler's plans for a campaign in the west.
After France had been defeated, Britain was the only nation able to fight Germany and was not prepared to deny Germany's dominance of the Continent: ‘Germany had a military position and sufficient freedom of action to make Britain's defeat inevitable, if not quick and easy. Considering the greater resources of German controlled Europe, Britain's position without outside help, was hopeless.’ However, as soon as the existence of Britain came under threat, Germany would have to reckon with the United States.