The Maginot line was constructed by france along the boarder with Germany in 1930s. It cosisted of hundreds of kms of long tunnels , concrete fortification , obstacles and was equipped with guns . It was mainly built to defend german attacks. During second world war germany attacked france which was an unexpected attack through the dense ardennes forest ,avoiding the maginot line which was a surprise to france . As they thought it was impossible to march through this dense forest. But the germans easily did that. The only obstacle they faced was because of there own traffic jams created by their long military convoys.