The Manstein Plan is one of the names used to describe the war plan of the German Army during the Battle of France in 1940. The original invasion plan was an awkward compromise devised by General Franz Halder, the chief of Oberkommando des Heeres staff that satisfied no one. Documents with details of the plan fell into Belgian hands during the Mechelen incident of 10 January 1940 and the plan was revised several times, each giving more emphasis to an attack by Army Group A through the Ardennes, which progressively reduced the offensive by Army Group B through the Low Countries to a diversion.
In the final version of the plan, the main effort of the German invasion was made against the Ardennes, the weakest part of the Allied line, where the defence was left to second-rate French divisions in the Second Army and the Ninth Army, on the assumption that the difficulty of moving masses of men and equipment would give the French plenty of time to send reinforcements if the area was attacked.