France's biggest failure was that they were still set on the idea that trench warfare was still the key to winning an engagement. And that's really all the Maginot Line was - a giant trench. The Germans had come to embrace mobile warfare, and by proxy the blitzkrieg. All they had to do was make a bit of an effort to push their way through the Ardennes Forest (no one thought they'd have the cajones to do so) and France was pretty much theirs.
