On this day in 1917, German troops begin a well-planned withdrawal—ordered several weeks previously by Kaiser Wilhelm—to strong positions on the Hindenburg Line, solidifying their defense and digging in for a continued struggle on the Western Front in World War I.
One month after Paul von Hindenburg succeeded Erich von Falkenhayn as chief of the German army's general staff in August 1916, he ordered 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. Its aim would be to hold the last line of German defense and brutally crush any Allied breakthrough 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.
In the wake of exhausting and bloody battles 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. The withdrawal to the Hindenburg Line meant that German troops were removed to a more uniform line of trenches, reducing the length of the line they had to defend by 25 miles and 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.
The German command correctly estimated that the move would gain them eight weeks of respite before the Allies could begin their attacks again; it also threw a wrench into the Allied strategy by removing their army from the very positions that British and French joint command had planned to strike next.
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 armies did not break it until October 1918, one month before the armistice.
Taken from: http://www.history.com/this-day-in-history/germans-begin-withdrawal-to-the-hindenburg-line [23.02.2015]