The document discusses the Maginot Line, a series of fortifications along France's border with Germany built between 1929 and 1940. It was intended to deter a German invasion but failed to stop the German advance in 1940. The Maginot Line was heavily armed and fortified, but the Germans were able to flank it by invading through Belgium and the Ardennes region instead of attacking it directly. This led to the rapid defeat of France in June 1940.