FALL GELB-1939
Hitler had always fostered dreams about major military campaigns to defeat the Western European nations as a preliminary step to the conquest of territory in the East, thus avoiding a two-front war.These intentions were realised in the Führer-Directive N°6. This plan was firmly based on the assumption that Germany's military strength could easily win a war in the West. Hitler ordered a conquest of the Low Countries: the Netherlands, Belgium and Luxembourg, to be executed at the shortest possible notice. It should prevent France from occupying them first, which would threaten the vital German Ruhr Area; it should also provide a basis for a successful long-term air and sea campaign against the United Kingdom. There was no mention whatsoever of a possible immediately consecutive attack to conquer the whole of France, although as much as possible of the border areas in northern France should be occupied.
Whilst writing the directive, Hitler had assumed that such an attack could be initiated within a period of at most a few weeks, but the very day he issued it he was disabused of this illusion. It transpired that he had been misinformed about the true state of Frances's forces.He largely ignored this advice calling it double agent lies and ordered plans to continue.
The evolution of German plans for Fall Gelb, the invasion of the Low Countries.
Halder's plan
On 19 March Franz Halder, chief of staff of the OKH, the Army High Command, presented the first plan for Fall Gelb ("Case Yellow"), the pre-war codename of plans for campaigns in the Low Countries: the Aufmarschanweisung N°1, Fall Gelb, or "Instruction for the advance Number 1, Case Yellow". Halder's plan has often been compared to the First World War Schlieffen Plan, executed in 1914. It was similar to it in that both plans entailed an advance through the middle of Belgium. But whereas the Schlieffen Plan had had a surprising gigantic encirclement of the French army in mind, aimed at a decisive victory, Aufmarschanweisung N°1 was based on an unimaginative frontal attack, sacrificing a projected half a million German soldiers to attain the limited goal of throwing the Allies back to the River Somme. Germany's strength for 1939 would then be spent; only in 1941 could France be totally beaten.
Hitler was very disappointed by Halder's plan. He had supposed the conquest of the Low Countries could be quick and cheap; but as it was presented, it would be long and difficult. Hitler agreed to the plan because of his belief that the German army should attack early, ready or not, in the hope that Allied unpreparedness might bring about an easy victory after all. He set the date on 1 May 1939. Because the plan as it was did not appeal to him, he tried to make it different, without clearly understanding in which way it could be improved. This mainly resulted in a dispersion of effort, since besides the main axis in central Belgium, secondary attacks were foreseen further south. On 29 October Halder let a second operational plan reflect these changes, Aufmarschanweisung N°2, Fall Gelb, which featured a secondary attack on the Liège-Namur axis.
Hitler was not alone in disliking Halder's plan. Gerd von Rundstedt, the commander of Army Group A, also disagreed with it. Unlike Hitler however, von Rundstedt, as a professional soldier, understood perfectly how it should be rectified. Its fundamental flaw was that it did not conform to the classic principles of the Bewegungskrieg, the "manoeuvre warfare", that had since the 19th century been the basis of German tactics. A breakthrough would have to be accomplished that would result in the encirclement and destruction of the main body of Allied forces. The logical place to achieve this would be the Sedan axis, which lay in the sector of von Rundstedt's Army Group A. Von Rundstedt on 21 October agreed with his chief of staff, Lieutenant-General Erich von Manstein, that an alternative operational plan had to be arranged that would reflect these basic ideas, making his Army Group A as strong as possible at the expense of Army Group B to the north.