Not listening to DeGaulle, France, 1939-40
Charles DeGaulle is not often considered to be a great military tactician, but in the decades between the World Wars, he and other generals argued for the deployment of tanks in columns separate from the infantry, to operate as horse-mounted cavalry had in earlier wars, cutting behind the flanks of the enemy. It was the same tactic the Germans used. Instead, French military doctrine was to use their tanks, some of which were more powerful than their German counterparts, as support for the infantry. The French planned a defensive war, anchored by the Maginot Line, and supported by the Ardennes, a region which they considered impenetrable.
In the event, German panzer columns sliced through the Ardennes, avoided the Maginot Line, and chopped the French formations to pieces. The French failed to launch counterattacks on the German columns because they lacked the formations with which to do so. Had DeGaulle, (and others, including American George Patton) adopted the use of tank divisions as fast offensive forces the result of the Battle of France in 1940 could have been very different. The failure to adopt DeGaulle’s thinking continued even after the Germans revealed the efficacy of their armored columns in Poland the preceding year.