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.
Infiltration tactics
Forms of these infantry tactics were used by skirmishers and irregulars dating back to classical antiquity, but only as a defensive or secondary tactic; decisive battlefield victories were achieved by shock combat tactics with heavy infantry or heavy cavalry, typically charging "en masse" against the primary force of the opponent. By the time of early modern warfare, defensive firepower made this tactic increasingly costly. When trench warfare developed to its height in World War I, most such attacks were complete failures. Raiding by small groups of experienced soldiers, using stealth and cover was commonly employed and often successful, but these could not achieve decisive victory.