In the 1920s, the lessons from WWI prompted France to protect her borders with an impenetrable wall on her eastern flank. Originally intended to run from the North Sea to the Mediterranean, the project left the Belgian border unprotected. Indeed, the wall failed to live up to its promise during the fighting of 1940. Having come through the war virtually intact, it now offers a fine example of modern fortifications, as do the forts of Guentrange, Hackenberg and Simserhof.