Who Developed the Concept of "Defense-in-Depth"?

One strategic thinker who without a doubt developed the strategy of defense-in-depth to its fullest extent is the German General Staff officer Colonel (later General) Fritz von Lossberg (1868–1942). Known in the German Army as the Abwehrlöwe, the lion of the defense, Lossberg played a key role in developing many of the principles and techniques modern armies apply during the conduct of defensive operations. As the chief of staff for five different field armies, he directed most of the major German defensive battles on the Western Front from the autumn of 1915 to the end of 1917.

He is credited, most notably, with the building of the Hindenburg Line, which was a focal point during the 1917 and 1918 campaigns. His experiences at the Somme contributed enormously to his defensive mindset and his creative  designs for field fortifications.  Our reviewer of his memoir, Lossberg's War, Terrence Finnegan (Link) outlined the influence of the 1916 campaign around the Somme on the general:

The assignment at the Somme is probably the most lucrative discussion from von Lossberg. He made many a personal reconnaissance of the region and provided in-depth views of the important battle. He saw the rolling terrain offering many possibilities for frontal and flanking observation and artillery fire. The Somme ground was also good for infantry defensive operations thanks to numerous solidly built-up settlements. Another insight was that the British and French had aerial superiority in this sector. They constantly flew over German-held terrain strafing infantry with machine gun fire from low altitudes and dropping bombs in the rear areas on any recognized movement. "Our flyers were powerless against this incredible superiority."  His superior work resulted in being awarded by the Kaiser the Orden Pour le Mérite, the first von Lossberg to be recognized with that honor from a long line of distinguished soldiers.

A 1939 article in Time, connected the concept of defense-in-depth with Lossberg's work on the Hindenburg Line.

Until 1917, each side attacked or defended linear fronts. In attack their tendency was to stretch and strain. On defense they tended to crack. Sent to the rear, Colonel Lossberg proceeded to construct a new kind of major fortification, based on zonal defense. He built what the Allies called the Hindenburg Line. It was not Hindenburg’s and it was not a line. The Germans called it the Siegfried Stellung (Siegfried Position).

Hindenburg Position East of Lens

The tactical principle of the Siegfried Position was that of defense in depth; the idea being that an offensive force may crack a narrow wall but will be stopped or bounced back by a series of cushions and springs backing up each other. Colonel Lossberg’s new type of front was some two miles deep, divided into forward zone, battle zone, rearward battle zone and two more rearward zones for mobile reserves.

Built of wire, wood, earth and some concrete, the Siegfried Position consisted of barbed wire entanglements, behind which came intrenchments and pillboxes connected with secondary intrenchments. Behind these were independent forts and strong points. From these, reserve troops, stationed far enough in the rear to be out of reach of enemy artillery, could be thrust out in any direction in counterattacks when the attacking enemy was exhausted by its advance. At this point the zonal defense system became an ideal means of launching a powerful offensive.

Sources: Military Review, Nov. 1917; Time magazine, 25 Sept. 1939