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Eingreif division (German: Eingreifdivision) is a term for a type of German Army formation of the First World War, which developed in 1917, to conduct immediate counter-attacks (Gegenstöße) against enemy troops who broke into a defensive position being held by a front-holding division (Stellungsdivision) or to conduct a methodical counter-attack (Gegenangriff) 24–48 hours later. 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.
During the Battle of the Somme (1 July – 18 November 1916), the use of defensive lines began to evolve into the defence of the areas between them, using the local troops of the trench holding divisions and Ablösungsdivisionen (relief divisions), held back beyond the range of Franco-British artillery, to replace front line divisions as they became exhausted. In the winter of 1916–1917, the use of such divisions and the fortified zones between trench lines was codified and divisions trained in the new defensive tactics. Training was based on the experience of the defensive battles of 1916 and the new principles of fortification, to provide the infrastructure for the new system of defensive battle by Stellungsdivisionen and Eingreifdivisionen (counter-attack divisions).
The new defensive principles and fortifications were used in 1917 to resist the Franco-British offensives. After failures at Verdun in December 1916 and at Arras in April 1917, the system of fortifications defended by Stellungsdivisionen supported by Eingreif divisions counter-attacking from the rear was vindicated during the French attacks of the Nivelle Offensive. The continuation of British attacks at Arras in the wake of the French debacle on the Aisne, led to the highest rate of casualties per day of the war but the system failed again at the Battle of Messines. German defensive tactics reached their ultimate refinement during the Third Battle of Ypres and there were only slight alterations during the defensive battles of late 1918. In the interwar period, German defensive thinking incorporated the new technology of aircraft, tanks, anti-aircraft guns and anti-tank guns in a much deeper defensive system filled with positions for delaying actions, before a counter-attack was made by armoured and mechanised formations.