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 Germany when they invaded through Belgium in 1940. The Maginot Line was heavily fortified with artillery, troops, and underground tunnels, but Germany was able to flank it by invading through the Ardennes forest. This led to France's rapid defeat in June 1940 and showed the limitations of the Maginot Line's defensive strategy.