In the 1920s, the lessons drawn from the First World War 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 was only realised in Alsace and Lorraine, leaving the Belgian border unprotected. However this fortification did not live up to its promise during the fighting of 1940. Having come through the war virtually intact, it now offers visitors a fine example of modern fortifications, as do the forts of Guentrange, Hackenberg and Simserhof.