Trench warfare
Trench warfare occurred when a revolution in firepower was not matched by similar advances in mobility, resulting in a grueling form of warfare in which the defender held the advantage. On the Western Front in 1914–1918, both sides constructed elaborate trench, underground, and dugout systems opposing each other along a front, protected from assault by barbed wire, mines, camouflaged trapping pits, and other obstacles. The area between opposing trench lines (known as "no man's land") was fully exposed to artillery fire from both sides. Attacks, even if successful, often sustained severe casualties.
Eingreif division
"Eingreif" division is a term for a type of German Army formation of World War I, which developed in 1917, which was responsible for engaging in immediate counter-attacks ("Gegenstoße") against enemy troops who broke through a defensive position being held by a front-holding division (). Attacks by the French and British armies against the "Westheer" on the Western Front had been met in 1915 and 1916 by increasing the number and sophistication of trench networks, the original improvised defences of 1914 giving way to a centrally-planned system of trenches in a trench-position and then increasing numbers of trench-positions, to absorb the growing firepower and offensive sophistication of the Entente armies.