'Gelb' was the German unrealised strategic plan for the invasion of the the Netherlands, Belgium, Luxembourg and France (19 October 1939/16 February 1940).

After the end of the German 'Weiss' (i) invasion and conquest of Poland in September 1939, which triggered World War II in Europe, there followed a period of inaction, called the Phoney War by the British, Drôle de guerre (funny war) by the French and Sitzkrieg (stationary war) by the Germans, in which none of the major protagonists undertook significant land operations. Adolf Hitler hoped that France and the UK would agree with his assertion that he had ended the period of Germany’s expansion in Europe, and thus make peace with Germany. On 6 October, he may have made limited peace overtures to both the Western powers. Even before they had time to respond, on 9 October, he signed a new military policy in case their reply was negative: this was the Führerweisung Nr 6 (Führer Directive No. 6).

Hitler had always entertained ambitions of major military campaigns to defeat the western European nations as a preliminary step to the conquest of territory in eastern Europe, thus avoiding a two-front war. However, these intentions were absent from Führerweisung Nr 6. This was based on the seemingly more realistic assumption that Germany’s military strength would still have to be increased for several more years before any such undertaking could be contemplated, and that in the shorter term only limited objectives could be envisaged for the improvement of Germany’s ability to survive an extended war in the west. Hitler ordered a conquest of the Low Countries to be executed at the shortest possible notice to prevent France from entering them and the British and French from basing bombers in these two countries to threaten Germany’s all-important industrial base in the Ruhr area. It would also provide the basis for a long-term air and sea campaign against the UK. However, there was no mention in Führerweisung Nr 6 of any immediate consecutive attack to take the whole of France, although as much as possible of the border areas in north-eastern France were to be seized.

Hitler initially proposed the start of the invasion of France on 25 October, but accepted that this date was probably too optimistic given the fact that 'Weiss' (i) had only just ended and the westward transfer of major forces could not be completed by this date. On 5 November Hitler proposed to Generaloberst Walther von Brauchitsch, the commander-in-chief of the army, that the offensive should start on 12 November, but von Brauchitsch rightly pointed out that the military had not recovered from the Polish campaign: most especially, the motorised formations had to recover their vehicles and repair the damage to them, and the forces in general had to rebuild their ammunition stocks, which were now largely depleted. von Brauchitsch offered to resign if Hitler did not change his mind: the offer was refused, but two days later Hitler postponed the attack, citing poor weather as the reason.

At this time the overall command of all the German armed forces was vested the Oberkommando der Wehrmacht. This was sometimes used by Hitler as an alternative army planning staff, but the direction of the offensive on the western front was the responsibility of the Oberkommando des Heeres, the army supreme command. While the army’s commander-in-chief was von Brauchitsch, the main responsibility for planning rested with General Franz Halder, the chief of the Oberkommando des Heeres’s general staff. Under the Oberkommando der Wehrmacht, the other service commands were the Oberkommando der Luftwaffe, led by Hitler’s close political colleague Generalfeldmarschall Hermann Göring, and the Oberkommando der Marine led by Grossadmiral Erich Raeder.

On 10 and 12 October 1939 the UK and France rejected Hitler’s peace overtures. With the prospect of war with the UK and France now a certainty, on 19 October Halder presented the first plan for 'Fall Gelb' (Case Yellow), the pre-war codename for the plans drafted for a campaign in the Low Countries as the Aufmarschanweisung Nr 1, Fall Gelb (Deployment Instruction No. 1, Case Yellow). Halder’s plan has often been compared to the Schlieffen plan, which the Germans attempted in 1914 in the opening phase of World War I. The two plans were certainly similar inasmuch as each was centred on an advance through the middle of Belgium, but while the intention of the Schlieffen plan was to gain a decisive victory by following this with a smart left wheel round the west of Paris to encircle the French army, Aufmarschanweisung Nr 1 envisaged a frontal attack, sacrificing a projected half million German soldiers to attain the limited goal of throwing the Allies back to the line of the Somme river. This would exhaust all of Germany’s available strength for 1940, and only in 1942 would it then be possible to launch the main attack to destroy France.

Hitler was unhappy with Aufmarschanweisung Nr 1, and initially reacted with the decision that the German army should attack at a time as early as possible, whether or not it was deemed ready, in the hope that the Allies would be unprepared and there would follow an easy German victory. There followed a series of postponements as senior commanders repeatedly persuaded Hitler to delay the attack for a few days or weeks in order that some critical defect in the preparations could be corrected, or to wait for better weather. Hitler also tried to alter the plan, which he found unsatisfactory, but without clearly understanding and recommending any way in which it could be improved. This had the result of creating a dispersion of effort: although the main axis would still be directed through central Belgium, secondary attacks would be undertaken on the flanks. Hitler made such a suggestion on 11 November, but on 29 October Halder had provided a second operational plan, Aufmarschanweisung Nr 2, Fall Gelb, which included a secondary attack on the Netherlands.

Hitler’s dislike of Halder’s plan was echoed by Generaloberst Gerd von Rundstedt, the commander of Heeresgruppe 'A', who saw that the plan did not adhere to the classic principles of the Bewegungskrieg (manoeuvre warfare) concept which had lain at the heart of German strategic thinking since the 19^th century. von Rundstedt firmly believed that the dictates of the Bewegungskrieg concept demanded that a breakthrough be effected to result in the encirclement and destruction of the main strength of the Allied forces. The most practical place to achieve this, von Rundstedt believed, was in the region of Sedan, which lay in the sector of his own army group. On 21 October, von Rundstedt agreed with his chief-of-staff, Generalleutnant Erich von Manstein, that there had to be an alternative operational plan which would reflect these basic ideas, making Heeresgruppe 'A' considerably stronger at the expense of Generaloberst Fedor von Bock’s Heeresgruppe 'B' farther to the north.

Halder’s Aufmarschanweisung Nr 2, Fall Gelb was based on the commitment of 102 divisions (including nine Panzer and six motorised), with emphasis again placed on a strong right wing to sweep through Belgium into the part of France to the north of the Somme river to take Boulogne and Dunkirk and so cut the shortest sea communication route between France and the UK.

The German right flank was thus to comprise von Bock’s Heeresgruppe 'B' reinforced to 43 divisions for a westward thrust through the area to the north and south of Liége in Belgium. Within Heeresgruppe 'B' the central force was to be Generaloberst Walther von Reichenau’s 6^th Army (containing five Panzer divisions) on the left and directed toward Ghent, and Generaloberst Günther von Kluge’s 4^th Army (containing four Panzer divisions) on the right and directed toward Thuin on the Sambre river.

Cover for this main offensive strength was provided in the north by General Georg von Küchler’s 18^th Army directed toward Antwerp and in the south by General Maximilian Reichsfreiherr von Weichs’s 2^nd Army directed toward Givet.

Farther to the south would be two more major formations. The more northerly of these was to be von Rundstedt’s 22-division Heeresgruppe 'A', which was to advance on a front between Laon and Longwy with General Wilhelm List’s 12^th Army on the right and General Ernst Busch’s 16^th Army on he left; and the more southerly was to be Generaloberst Wilhelm Ritter von Leeb’s 18-division Heeresgruppe 'C', which was to operate defensively on a front between Longwy and the Swiss frontier with General Erwin von Witzleben’s 1^st Army on the left and General Friedrich Dollmann’s 7^th Army on the right.

Some 19 divisions, including two motorised divisions, would be retained by the Oberkommando des Heeres as a general reserve.

This overall plan was orthodox in the extreme, and the French 'Dyle' plan was indeed schemed to counter just such an offensive. Hitler accepted this plan on 29 October 1939, and the only modification made in the short term was the inclusion of the Netherlands as a country to be conquered. The plan originally called for the 18^th Army and 6^th Army to cross the 'Maastricht appendix' of the southern Netherlands en route to their objectives in Belgium, the Germans hoping that the Dutch would not enter the war for this 'small' violation of their territorial integrity.

Hitler’s second-in-command and commander-in-chief of the Luftwaffe, Göring feared that any such German move might persuade the Dutch to accede to British pressure for the forward basing of British bombers on Dutch airfields, however, so it was decided to extend the scheme northward to include the whole of the Netherlands.

By the beginning of November Hitler too was starting to have doubts about the wisdom of leaving Heeresgruppe 'A' so weak, and demanded that General Heinz Guderian’s XIX Corps (mot.) (two Panzer divisions, one motorised division and two SS motorised regiments) be redeployed to Heeresgruppe 'A' in order that it could drive through the Ardennes to take Sedan on the Meuse river as an offensive bridgehead from which major armoured operations could be launched in the centre should the armoured thrust in the north fail.

von Manstein was already thinking along similar lines, but his plans had proceeded no further than the Oberkommando des Heeres, and it was only after Hitler’s intervention that the scene was set for the evolution of the definitive 'Sichelschnitt'.