The Maginot Line was a really long wall that France built a long time ago to keep bad guys out. It was underground, too. Think of it like a big, secret fort that the French made to keep their country safe.
But bad guys can be tricky, and they didn't want to just walk right up to the wall and knock on the door. So they came up with a sneaky plan. They decided to go around the wall instead.
The French didn't expect this, so when the bad guys came knocking on the back door, they were caught off guard. The wall didn't help them anymore, and their country was invaded.
So, the lesson here is that sometimes things aren't as they seem. Just because you have a big wall, it doesn't mean you're completely safe. You always have to be alert and ready for anything.