In March 1917 a portion of the German army strategically withdrew behind a prepared defensive position known as the Siegfriedstellung or Hindenburg Line. This 90 mile long fortification had been constructed from late 1916. Devastation of the vacated land was designed to hamper the Allied advance.
Details of these defences around the town of St. Quentin became known to the Allies through captured German maps of February 1917. This valuable information was overprinted onto these Ordnance Survey maps one year later.
By that time, Allied attacks at Bullecourt and Cambrai had failed to break the Hindenburg line. The captured information explained why. It showed strong defences: primary and secondary infantry lines. Ovals marked the barrage areas, red shaded areas the focus of field artillery, and the positions of field guns, howitzers, anti-aircraft and naval guns.