Maginot Line
The Maginot Line was an impregnable line of fortifications built by France after World War I to prevent any future German invasion. It consisted of gun emplacements, underground tunnels, minefields, and other obstacles meant to slow the Germans, cause heavy casualties, and allow the French time to mobilize and counterattack.
In World War II, the Germans simply detoured around it to the north, bypassing the line with an attack through Belgium. However the objective of the construction of the line was met, and even when attacked from behind, the French garrison was able to hold out for significant periods of time. It was only through a failure of the mobile reserve to contain the blitzkrieg that France was forced to surrender.