The German Blitzkrieg of France was conceptual genius. (codenamed Operation Sichelschnitt)
France knew Germany were going to attack them, so they created a massive fortified line from the South coast to some impassible mountains, to stop the Germans being able to attack from there. Above the impassable mountains were Belgium, Luxemburg and Holland, so the French kept a large force pointed in that direction, if Germany came that way they would have to invade those countries, get all their supplies and munitions through them, and France would have time to form a defencive line.
Germanys generals knew exactly what the French were doing, so they built up forces beside Belgium, and invaded them, as a distraction but at the same time had a large motorised force gunning it through the impassable mountains:- a manoevre thought impossible. They appeared behind enemy lines, and stormed through the countryside, getting about halfway to Paris before word got to the French generals that they were under attack.
It caught everyone by surprise, and cemented the idea of the previously formidable French army as being surrender monkeys, for over half a century and beyond.
