Overview of the Plan 
Developed by German Generalleutnant Erich von Manstein, the plan greatly modified the original 1939 versions by Franz Halder of the invasion plan known as Fall Gelb. One way to look at the Manstein Plan was that it was the German Army's answer to the French Army's Dyle Plan. Originally, in Aufmarschanweisung N°1, Fall Gelb, the German Army planned to push the Allied forces back through central Belgium to the Somme river, in northern France, not unlike the first phase of the famous Schlieffen Plan of the First World War. However, on 10 January 1940, the Mechelen Incident took place: a German aircraft carrying documents containing parts of the operational plans of Fall Gelb crashed in Belgium, thus prompting another review of the invasion plan. While Fall Gelb was revised by Halder, not fundamentally changing it in Aufmarschanweisung N°3, Fall Gelb, Von Manstein was able to convince Hitler in a personal meeting on 17 February that the Wehrmacht should attack through the Ardennes forest, followed by a strategic drive to the coast.
Details of the Plan 
Von Manstein, chief of staff of Army Group A, had originally formulated his plan in October 1939 in Koblenz on instigation of his superior General Gerd von Rundstedt, who rejected Halder's plan, both because of professional jealousy and because it wouldn't lead to a decisive victory over France. Von Manstein's first thoughts were rather traditional, envisaging a swing from Sedan to the north to obliterate the Allied armies in a classical Kesselschlacht or annihilation battle. When discussing his intentions with Lieutenant-General Heinz Guderian, commander of Germany's elite armoured corps, the latter proposed to turn it into a more "Fullerite" strategy by avoiding the main body of the Allied armies and swiftly advancing with the armoured divisions to The Channel instead, to cause a collapse of the enemy by catching him off guard and cutting off his supply lines. It was thus Guderian who introduced the true "Blitzkrieg" elements to the plan, while Von Manstein had at first many objections against this aspect, especially fearing the long open flank created by such an advance. Guderian managed to convince him that the danger of a French counterattack from the south could be averted by a simultaneous secondary spoiling offensive to the south, in the direction of Reims. Guderian before the war had generated much interest for the theories of John Fuller, though never fully endorsing them.
When Von Manstein first presented his ideas to the OKH, he didn't mention Guderian's name and made his classical swing to the north the main effort, while a limited number of armoured divisions protected the left flank of this movement, acting in a classical cavalry strategic reconnaissance rôle. These changes didn't reflect a change of mind on his part, but were thought necessary by him because the original conception was too radical to be acceptable and many conservative generals considered Guderian himself as too radical also. His views were flatly rejected by Halder and Walther von Brauchitsch however. Reformulating them in a more radical sense didn't help and late January Halder managed to remove Von Manstein to the east by having him promoted commander of XXXVIII Army Corps. Von Manstein and Halder were old rivals: in 1938 Von Manstein had been the successor of chief of staff Ludwig Beck but had been removed from this position when the latter fell into disgrace with Hitler because of the Blomberg-Fritsch Affair. On 1 September 1938, Halder, not Von Manstein, had replaced Beck.
However two officers of Von Manstein's staff, Lieutenant-Colonel Günther Blumentritt and Major Henning von Tresckow, were outraged by Halder's behavior. Late January they contacted Hitler's personal Army attaché, Lieutenant Colonel Rudolf Schmundt (an old acquaintance of von Tresckow) when he was visiting Koblenz, who informed Hitler of the affair on 2 February. Hitler, having found Halder's plans unsatisfactory from the very beginning, ordered on 13 February a change of strategy in accordance with Von Manstein's ideas, even after having only heard a rough outline of them. The general was invited to the Reichskanzlei in Berlin to explain his plans in person to Hitler on 17 February, during a working lunch in the presence of Alfred Jodl and Erwin Rommel. Though Hitler felt an immediate antipathy against Von Manstein, for being too arrogant and aloof, he speechlessly listened to his argumentation, becoming very impressed by Von Manstein's logic. "Certainly an exceptionally clever fellow, with great operational gifts, but I don't trust him", Hitler remarked after Von Manstein had left.
Halder now had to make a fourth main version of the attack plan, Aufmarschanweisung N°4, Fall Gelb. Von Manstein would not be further involved in the planning process, returning to his command of the Army Corps. This new plan conformed to Von Manstein's proposal in this respect that Army Group A would provide the central thrust of the invasion through the Ardennes in southern Belgium. After crossing the Meuse River between Namur and Sedan, Army Group A would then swing northwest towards Amiens, while Army Group B executed a feint attack in the north to lure the Allied armies into the trap. However in many ways the plan was fundamentally changed by Halder. It no longer envisaged a simultaneous secondary attack to the south. Also, the "Blitzkrieg" elements were largely removed. The river crossings were to be forced by infantry and there would be a long consolidation phase during which a large number of infantry divisions would be built-up in the bridgeheads. The armoured divisions should then advance in a coherent mass together with the infantry divisions. There would thus not be an independent deep strategic penetration by the German armor.
Executing the Plan 
In reality however, Guderian and the other panzer generals, Rommel among them, would simply disobey orders and advance to The Channel and further to the coastal French towns of Calais and Dunkirk as fast as they could without waiting for the infantry, only temporarily halted by Hitler's orders on 17, 22 and 24 May. The effects of the Manstein Plan were devastating for the Allied armies, as they were effectively encircled by Army Groups A and B, thus sparking a desperate evacuation from Dunkirk. The losses in the north and resulting lack of mobile reserves led to the defeat of the remaining French forces and Germany's complete victory over France.
This resounding success came as a complete surprise even to the Germans, who hardly had dared to hope for such an outcome. Most generals had vehemently opposed the plan as being much too risky; even those supporting it had mainly done so out of desperation, because Germany's geostrategic position seemed so hopeless. Count Ciano later in the war observed that "victory has a hundred fathers, but defeat is an orphan", and Fall Gelb would have no lack of sires. The two most prominent among them would be Hitler himself and Halder. Because Hitler hadn't liked Halder's original plans, he had suggested many alternatives, some of them bearing some resemblance to the Manstein Plan, the closest a proposal made by him on 25 October 1939. Soon Nazi propaganda began to claim that the victory was a result of Hitler's military genius; Hitler praised Von Manstein with the words "Of all the generals, with whom I spoke about the new attack plan in the West, Manstein was the only one who understood me!". Halder after the war claimed he was the main inventor, supporting this with the fact that he had begun considering to change the main axis to Sedan even before 13 February — indeed as early as September 1939 — and that Von Manstein's original proposal was too traditional.
The Manstein Plan is often seen as either the result of, or the cause of a mid-twentieth century Revolution in military affairs. In the former hypothesis, expounded by Fuller and Basil Liddell Hart immediately after the events, the Manstein Plan is presented as a natural outcome of deliberate changes in the German military doctrine during the twenties and thirties by men as Guderian or Hans von Seeckt implementing Fuller's or Liddell Hart's ideas. Thus an explicit "Blitzkrieg-doctrine" would have been fully established by 1939 of which the Manstein Plan was but the most spectacular implementation and the Invasion of Poland an earlier application. The doctrine would have been reflected in the organisation and equipment of the German Army and Airforce and would have been radically different from the obsolete doctrines of France, Britain and the Soviet-Union — except for the contributions of some farsighted individuals in these countries such as Mikhail Tukhachevsky and Charles de Gaulle and of course Fuller and Liddell Hart themselves. That the earliest plans by Halder or Von Manstein and the final plan by Halder did not conform to this doctrine is then seen as an anomaly, to be explained by special circumstances.
In the latter hypothesis, promoted by Robert Allan Doughty and Karl-Heinz Frieser, the Manstein Plan is instead a return to the classic principles of the 19th century Bewegungskrieg but now radically adapted to the full potential of modern technology by a sudden and unexpected departure from established German doctrine through the Blitzkrieg-elements provided and executed by Guderian. It claims that the influence of Fuller and Liddell Hart in Germany was limited and much exaggerated by the two writers and that no explicit true Blitzkrieg-doctrine can be found anywhere in the official pre-war German army documentation. It finds further support in the fact that German tank production had no priority and that the plans of the German war economy were at first based on the premise of a long protracted war, not on the expectation of swift victories. The hypothesis allows for a gradual implementation during the thirties of technological advances in a shared moderated Bewegungskrieg doctrine used in all major powers prior to 1940, with more subtle differences between the nations. The Invasion of Poland would then not yet be a true Blitzkrieg campaign, but a classic annihilation battle instead. The lack of Blitzkrieg elements in the official German plans for Fall Gelb is seen as the normal and expected outcome of this situation. Only after the sudden success of the radical execution of the Manstein Plan by Guderian would Blitzkrieg have been adopted as an explicit doctrine, in this view making Operation Barbarossa the first deliberate Blitzkrieg campaign.
Guderian himself, who in both hypotheses plays a key rôle, presented the situation in his postwar book Erinnerungen eines Soldaten (literally "Memories of a Soldier" but translated under the title Panzer Leader) as basically conforming to the second hypothesis, with him being a lone voice struggling against the resistance by a reactionary majority of the German officer corps.
Summarizing the Plan 
It is not uncommon in the literature to call the Manstein Plan Operation Sichelschnitt and this had led to the misunderstanding that this was the official name of the entire plan or at least of the attack by Army Group A. The official name however was Aufmarschanweisung N°4, Fall Gelb as issued on 24 February 1940 and the suboperation through the Ardennes had no special designation. Sichelschnitt is nothing but a literal German translation of "Sickle Cut", a catchy expression used after the events by Winston Churchill. After the war it would be adopted in the writings of the German generals.
See also 
- Julian Jackson, The Fall of France: The Nazi Invasion of 1940 (New York: Oxford University Press, 2003), p.30.
- Karl-Heinz Frieser, Blitzkrieg-Legende, p. 81
- Diary (1946) Vol. 2, 9 September 1942: La Vittoria trova cento padri, e nessuno vuole riconoscere l'insuccesso
- Karl-Heinz Frieser, Blitzkrieg-Legende p. 92
- Franz Halder, Hitler als Feldherr, 1949, p.28
- Maurice Vaïsse e.a., Mai-juin 1940, Défaite française, Victoire allemande - Sous l’oeil des historiens étrangers Autrement - Mémoires 2000