History.com: Hindenburg gives order to strengthen German defenses:
On September 16, 1916, one month after succeeding Erich von Falkenhayn as chief of the German army’s general staff during World War I, General Paul von Hindenburg orders the construction of a heavily fortified zone running several miles behind the active front between the north coast of France and Verdun, near the border between France and Belgium.
This “semi-permanent” defense line, as Hindenburg called it, would be the last line of German defense; its aim was to brutally crush any Allied breakthrough on the Western Front in France before it could reach the Belgian or German frontier. The British referred to it as the Hindenburg Line, for its mastermind; it was known to the Germans as the Siegfried Line.
After waging exhausting and bloody battles against the Allies at Verdun and the Somme, and with the U.S edging ever closer to entering the war, Germany’s leaders looked to improve their defensive positions on the Western Front. In February 1917, the German army began a well-organized withdrawal to the Hindenburg Line, a move calculated to give a period of respite before the Allies could begin their attacks again. The withdrawal reduced the length of the line the Germans had to defend by 25 miles, freeing up 13 army divisions to serve as reserve troops. On their way, German forces systematically destroyed the land they passed through, burning farmhouses, poisoning wells, mining abandoned buildings and demolishing roads.
After the withdrawal, which was completed May 5, 1917, the Hindenburg Line, considered impregnable by many on both sides of the conflict, became the German army’s stronghold. Allied troops would not breach it until the last days of September 1918, barely one month before the armistice.