The defensive doctrine upon which France had built the Maginot Line wrongly assumed that an attack by Germany would be from the front as had been common in the Great War. The Ardennes was considered to be terrain that an attacker would not attempt. France had not fortified the Belgian route and by fienting at the fortified front and attacking through the Ardennes and the Low Countries, the German's Second Army Group was able to penetrate France in depth. The static defense had failed.