Originally, the OKW, Oberkommando der Wehrmacht (High Command of the Wehrmacht), envisaged an attack from the western front by advocating a strategy of enveloping the Allied armies from the north; in a way a revival of the Schlieffen plan of 1914 which would have led a powerful army group B, of General Fedor von Bock, stationed in the north of the German front, to overrun the Franco-Anglo-Belgian units on their left wing, by a armored offensive through Belgium and the Netherlands, and to bring back the defeated elements on the Lorraine region. Then, in a second step, it was planned to take the remaining Allied troops in a pincer movement; army group B coming from the west pushing them back on general von Rundstedt's army group A, placed facing the northern region and Alsace, and which would have played the role of an anvil on which definitively crushed the best allied divisions.
However, following the crushing of the Polish forces in less than a month, and having realized the remarkable tactical value represented by the tank-aircraft tandem, Hitler, who still hoped to find a political settlement to the ongoing conflict with the West, kept postponing the date of the start of operations in the West. This punctual respite allowed General von Manstein to submit a new plan to him. This one, baptized by Winston Churchill “Sichelschnitt” (scythe blow), took the opposite of the previous theory and advocated an attack in force coming, no longer from the north, but from the center. He started from the hypothesis that it was necessary to surprise the adversary in the absence of the cuirass then, the surprise passed, to overtake him in a rapid advance towards the English Channel:the pivot of the offensive could only be found at through the wooded massif of the Ardennes, a region defended by French units of poorly armed and under-equipped reservists and the precise place where the construction of the Maginot Line had been stopped. This new plan, by its very boldness and its tactical as well as strategic logic, excited Hitler, who imposed it on a reluctant OKW.
From then on the Fall Gelb was born; henceforth the weight of success rested on the center army group, army group A, whose operational capacities were hastened to be reinforced by placing at its disposal two-thirds of the armored forces of the entire army.