Why did French commanders consider the Ardennes Forest difficult for tanks to cross?

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Nazi leaders in Paris — Battle of France Adolf Hitler (center) in front of the Eiffel Tower in Paris shortly after signing an armistice with France, June 1940.

French commanders considered the Ardennes Forest difficult for tanks to cross because they deemed the region impassable to armored vehicles due to its hilly and dense terrain.1

Because of this belief, the French military viewed the Ardennes as a natural barrier and consequently left that sector of the frontier as the least-fortified stretch of their entire defensive line. This strategic miscalculation proved fatal; the German army successfully moved their tank columns through the forest by utilizing available roads and having infantry divisions march across country using field and woodland paths, allowing them to reach the Meuse River with remarkable speed.1