The Magno Line and the French Army in the First World War

There were serious problems with French demographics and geography that required some way of holding on to as much French territory as possible. Attempts to solve this problem in the first world war was a quick and decisive offensive and it had utterly failed so defense was seen as the answer. The Magno line was not designed to completely prevent an invasion even when they were expanded onto the French and Belgium border. They were meant to make it very costly and very time consuming for a German attack to push through them. Being able to dictate the route of your enemy's advance is an important advantage.