It takes time, and men, like any other fortification.
A US example
On 25 November the 6th Armored Division and its attached battalions from the 134th Infantry were in position to begin what was expected to be the last phase in the attempt to reach and cross the Maderbach. The weather had become progressively worse and the armor was roadbound in consequence. Indeed, even the roads presented a problem. Near the frontier they were more poorly constructed, cratered by demolitions, and interdicted at frequent intervals by antitank ditches. In addition the advance was entering the old fortified zone of the Maginot Line, which though no longer a first-class military barrier provided gun emplacements, pillboxes, and antitank obstacles to slow down the attack. Under these conditions the armor could do little toward carrying the assault. In slow and painful progress the infantry, both armored and attached, had to fight to clear every foot of road, as well as to establish "bridgeheads" wherever an antitank ditch intervened. As a result the number of sick and combat fatigue cases mounted rapidly, even though officers did all that they could to provide dry socks and warm clothing for their men. Rifle strengths dwindled; as one example, Combat Team Britton (the 9th Armored Infantry Battalion reinforced) lost only four killed and two wounded on 25 November, but found it necessary to evacuate twenty-six sick and ninety-three combat fatigue cases.