In the years before World War I, the French had constructed very strong and modern fortresses along their Eastern border. During the Battle of Verdun, France's brave soldiers were ultimately pushed out of their own fortress, Douaumont. Once out, however, the French realized just how well they had built their own fortress because they found it almost impossible to get back in. Year later, when France was constructing its Maginot Line, the walls facing France were purposely left thinner than those facing Germany. In the event that the modern fortress fell, the French could more easily fight their way back in through the weaker walls. A valuable lesson learned from Douaumont.