Hindenburg/Siegfried Line WW1

The Hindenburg Line - Ludendorff's Defence In Depth - WW1 > .
The Hindenburg Line Breaks - The Lost Battalion Returns - WW1 > .
The Battle of the Selle - Ludendorff Resigns - WW1 > .

1916, the Year of Battles, had strained Germany's resources everywhere but especially on the Western Front they needed to defend their captured territory against an ever growing number of Entente Forces. Erich Ludendorff decided to shorten the front line where possible and built a new "Defence In Depth" line: The Siegfried Line. This defensive network was supposed to grind the Entente forces down while freeing up more German resources.

The German blitzkrieg was pioneered by Ludendorff on the Western Front in 1917 and 1918, where it involved the abandonment of lines and their replacement with ‘blobs’: small groups of specialist troops forming either a firebase (in defence) or an ‘infiltration’ force (in attack). Since then, the effectiveness of offensive tactics had been greatly enhanced by the development of armour, motorised infantry, and air-power. This made possible a qualitative shift from mainly static, defensive warfare to a fluid, fast-moving war of movement.

https://www.military-history.org/books/new-ways-of-war-by-tom-wintringham.htm .

Field Fortifications - Defense in Depth ..