Trench warfare
Trench warfare prevailed on the Western Front from late 1914 until the Germans launched their Spring Offensive on 21 March 1918. After the buildup of forces in 1915, the Western Front became a stalemated struggle between equals, to be decided by attrition. Frontal assaults, and their associated casualties, became inevitable because the continuous trench lines had no open flanks. Casualties of the defenders matched those of the attackers, as vast reserves were expended in costly counter-attacks or exposed to the attacker's massed artillery. There were periods in which rigid trench warfare broke down, such as during the Battle of the Somme, but the lines never moved very far. The war would be won by the side that was able to commit the last reserves to the Western Front.
Trench warfare
As far back as the 18th century, Prussian military doctrine stressed maneuver and force concentration to achieve a decisive battle (). The German military searched for ways to apply this in the face of trench warfare. Experiments with new tactics by Willy Rohr, a Prussian captain serving in the Vosges mountains in 1915, got the attention of the Minister of War. These tactics carried Prussian military doctrine down to smallest units — specially trained troops maneuvered and massed to assault positions they chose on their own. During the next 2 years the German army tried to establish special stormtrooper detachments in all its units by sending selected men to Rohr and have those men then train their comrades in their original units.