From what I gather, the Mechelen Incident is what prompted German High Command to switch to the Manstein Plan since they were spooked that the original Fall Gelb had fallen into Allied hands, and that had the incident not occurred they would have activated what amounted to Schlieffen 2.0 – which is exactly what Britain and France were expecting! Is this correct? If so, could the absence of a "sickle cut" behind Allied Lines have prevented the Fall of France? What would the implicatiions for this be around the world?
Mays in 'Strange Victory' is good English language description of the development of the German plans for attacking in the west. The short version is Mays describes a evolutionary development over five to six months of frequent map and field exercises. These repeated tests of many options led to the Sickle Cut plan in its final form.
Mays gives Manstein credit for planning
one of many plans for placing the 'schwerpunckt' or main effort in the Ardennes, however that concept did not originate with him. It dates to a requirement from Halder for a early November 1939 map exercise run at the Zossen HQ facility. The advance to the Channel & the massing of multiple armored corps into a single operational entity emerged gradually over the winter, the most successful technique of many tried in the map and field exercises. Mansteins plan, written for Army Groups A role in the November 39 Zossen war game had the armored corps separated & still operating with the field armies as they had two months earlier in Poland.
Mays credits the Melichin Incident as the final event that led to the cancellation of the winter offensives Hitler had previously been in love with. He abruptly reversed his attitude and went from pressing Halder & the others to getting a attack underway immediately. Hitler ordered a fresh start & new plans, but Mays points out that Halder simply continued the developing the several plans on the table. Refining each as the optimal plan emerged.
Memos or reports on these exercises show that Halder & others felt the best results were when the main effort was in the Ardennes. However there were pressures from other directions to weaken that. The Air Force wanted the Dutch airfields for attacking Britain. Goering & his staff had no confidence the army would secure and airfields in western Belgium or France suitable & they argued for a right wing strong enough to capture the airfields they needed. Hitler was mercurial as always favoring one plan in the morning, and prohibiting it in favor of another in the evening. The logisticians pointed out correctly Army Group A could not be supplied through the Ardennes. Securing the rail network passing through Leige & across the Belgian plain was essential.
While Halder felt the Ardennes option had the best potential this was not realized in the regular map exercises. While tactical and operational successes were gained none of the plans produced a decisive result. In the end the German armies were stalled in Belgium or other France & the 'Panzerwaffe' so successful in Poland ground down to ineffectiveness. That changed in the March 1940 wargame. The intelligence officer running the French army proposed delaying French actions by 48 hours. He argued defects in the staff and command of the several army groups, and Georges HQ (the North Front) would slow French reactions drastically. Previously the French on the map had been run at what was thought to e normal speeds in terms of Observation, Decision, Action. Not unexpectedly allowing for a slower French reaction resulted in a desire result (successful breakout & exploitation by Kliests armored group). Most of the Generals and staff observing this exercise judged the result unrealistic. A German wank that would never occur on the battlefield. A few like Guderian & Kliest saw it as validating their arguments for the Sicklecut maneuver. Harder who in six+ months of testing plans had seen nothing better &continued with the plan as it has evolved.
One other point that Mays does not much address is the fact that the Channel dash was not the only option on the table for the exploitation. A exploitation to Paris was seriously considered and tested. Guderian in his memoir 'Panzer Leader' describes how as late s the 14th May he was unsure if his corps would be ordered to attack to the Channel, or Paris. Kleist told him the Channel & Guderian was off. This late decision for the exploitation target is discussed by a few others as well.
Getting back to this:
From what I gather, the Mechelen Incident is what prompted German High Command to switch to the Manstein Plan since they were spooked that the original Fall Gelb had fallen into Allied hands, ...
Aside from Mays, other historians like Horne of Chapman date the favor for the main effort in the Ardennes far earlier than January 1940. The idea that this January event dates a whole sale & radical change in plan seems to come from Hitler histrionic fit after the Mechelen Incident. His frequent nervous changing of favor from one plan to another confuses that at OKW there was a steady methodical development of multiple plans. It is correct that as a result of Hitlers post Mechelen order Halder ordered a top to bottom review of the plans, but the result was little different than previous. ie: one January proposal was Holland be excluded from attack, left neutral. This would help strengthen the attack further south. The Luftwaffe representatives blackballed this, insisting the Dutch airfields were essential. So nothing changed there. The armored group attacking thru the Ardennes before the Mechelen Incident had already been expanded. Guderian, & others, even Rundsteadt had been arguing before January for again strengthening that armored group.
So much about the development and execution of the Sicklecut plan in the books derives from recycling of third and fourth hand sources trying back to post war statements. Often statements not checked or by people speaking out of context. Mays went to the trouble of reviewing the surviving documents from the OKW, Halder & staff. Those show a much more evolutionary development by methodical Prussian staff work. Mansteins role was relatively early in this & minor. Mainly as the Army Group A Chief of Staff he did the research and investigation that showed a modern AG could advance rapidly through the region. Solid and credible work expected of the German General Staff Officers.