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.
Military history of Germany
By 1917 the German army had begun employing new infiltration tactics in an effort to break the trench warfare deadlock. Units of stormtroopers, were trained and equipped for the new tactics, and were used with devastating effect along the Russian front at Riga then at the Battle of Caporetto in Italy. These formations were then deployed to the Western front to counter the British tank attack at the Battle of Cambrai.