Why were French defenses near Sedan vulnerable to the German attack?

The French defenses near Sedan were vulnerable primarily due to a rigid defensive strategy and a critical miscalculation regarding the terrain. French military leadership held a firm, erroneous belief that the dense, hilly Ardennes Forest was impassable to tanks, leading them to treat this sector as the least-fortified stretch of the entire front.12

Because of this assumption, the French 2nd and 9th armies stationed there were poorly equipped, lacking sufficient antitank weapons and antiaircraft artillery to counter a modern armored assault. Furthermore, the area was manned by troops of lesser quality compared to those stationed along the main Maginot Line. When German forces successfully navigated the Ardennes and launched a massive air bombardment with Stuka dive-bombers, the French defenders—already lacking the necessary defensive hardware—suffered a collapse in morale, allowing the Germans to secure a decisive bridgehead across the Meuse River.12