 The main construction was largely completed by 1939, at a cost of around 3 billion French francs.The line stretched from Switzerland to Luxembourg and a much lighter extension was extended to the Strait of Dover after 1934.The original construction did not cover the area ultimately chosen by the Germans for their first challenge, which was through the Ardennes in 1940, a plan known as "Fall Gelb" (Case Yellow), due to the neutrality of Belgium.