The Maginot Line depended on Belgium and Allied timing

The Maginot Line did what a fortified barrier was supposed to do by discouraging a direct German attack through its strongest sector. The wider French plan, however, assumed Belgian cooperation and enough time for French and British mobile forces to move forward to prepared river positions. Belgian neutrality and the collapse of earlier defense arrangements delayed that movement until after Germany crossed the border. The best Allied mobile formations then moved north while weaker troops held the Ardennes, allowing the German penetration to threaten their supply and retreat routes. The discussion therefore treats the 1940 defeat as a failure of the complete diplomatic and operational system, not proof that fortifications were inherently useless.