In 1939 Hitler assured Brauchitsch that England and France would not declare war when Germany moved against Poland. The general staff expected a war … - Adolf Heusinger

The general staff was of the opinion that a French attack in the west would have broken through, as our fortifications were not complete, nor were they in 1939, when the French could have broken through, although at heavier cost. The West Wall was completed only from Trier south to the Rhine in 1939. Northward it was incomplete and without any depth. After 1940 construction ceased. To a certain extent the West Wall was a bluff, like the Atlantic Wall. With regard to the latter it was impossible to fortify the entire coast and every military man must have concluded that a landing and a penetration of five kilometers would end all difficulties as far as fortifications were concerned.

The first plan for an offensive campaign [in the West] was formulated in November 1939. In substance it was a repetition of the 1914 Schlieffen plan. As the start of the campaign was delayed, doubts arose as to the achievement of any surprise with this plan. The basic idea of the new plan—the breakthrough in the Ardennes, crossing of the Meuse, and the trapping of British, French and Belgian forces in the north by pushing the tank forces through to the Channel—came to several minds at once. And in justice it should be said that one of these was Hitler’s. However, General Manstein, then chief of staff to Marshal Rundstedt, deserves the greater credit. He worked out the plan and proposed its adoption. The order was given to the Operations Division in February 1940 to replan the campaign along the proposed lines. From February 1940 to May 1940 the plan was subject to the sharpest criticism. Among the critics was General Guderian, who described the plan as a ‘crime against Panzers’. General Halder deserves the credit for defending the plan against all critics and insisting upon its execution. General Bock was also opposed to it and appealed to the chief of the general staff. Halder said once that he would stick to the plan if the chances of succeeding were only ten percent.

All equipment of the [British] expeditionary force was lost, and we knew that there were few reserves of men and materiel in the homeland. Never in modern times had Britain been in a more critical situation. Only a man like Winston Churchill could have brought the country through such a crisis. We had no plans for an invasion and no equipment and specially trained forces with which to undertake the invasion. Hence the delay, the hesitation, and finally Hitler’s decision not to risk it. Whether we should have risked it is of course now only a matter of historical interest. Admiral Wagner, with whom I have discussed this question recently and who was then chief of Naval Operations, is of the opinion that it would have failed. I think it could have been done. Militarily, this was for us one of the lost opportunities of the war. With regard to the air attack in August and September —the Battle of Britain—I can speak only from the standpoint of the army. It was not thought possible to conquer Britain from the air. The objective was to destroy British air power and gain control of the air. This failed. English aircraft were greater in number than estimated or Britain’s production was higher than estimated. By the middle of September it was obvious that the attack against London would not be decisive. Our losses in aircraft from improved flank and other defense measures became too high in proportion to results achieved. The air attacks were then switched to new objectives—the production and armament plants became targets with a view of knocking out or delaying British rearmament. But in my opinion these were only substitute objectives fixed after the failure to achieve the first main objective—to destroy the British air power and gain control of the air over London and the south coast.