Dense Ardennes forest was assumed impassable to armies before 1940

Evidence

Luckily, these were Ardennes kilometers, allergic to armies, as anyone could see: in 1914 Joffre had come to grief over them and the lesson had not been wasted. Grange observed with satisfaction the enormous green stain which thrust its tentacles from Liége to beyond the Meuse; as far as forests went, it was about as serious an obstacle as you could find — Julien Gracq, Balcony in the Forest, 229

Categorization

• corroborated: 2 independent passages support this claim — Julien Gracq, Balcony in the Forest, 229