British soldiers attacking the Hindenburg Line, World War I, 1917-1918.Artist: Realistic Travels Publishers

British soldiers attacking the Hindenburg Line, World War I, 1917-1918. Soldiers bombing Germans out of their deep dug-out with grenades. The Hindenburg Line was a vast system of defences built on the Western Front by the Germans in the winter of 1916-1917. It extended from Lens in the north to beyond Verdun and was believed by the German High Command to be impregnable. In fact British and Newfoundland forces using tanks briefly broke through the line at Cambrai in 1917, and it was breached in several places in September 1918 during the Allied Hundred Days Offensive. Stereoscopic card detail. (Photo by The Print Collector/Print Collector/Getty Images)