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
Artillery dominated the battlefields of trench warfare. An infantry attack was rarely successful if it advanced beyond the range of its supporting artillery. In addition to bombarding the enemy infantry in the trenches, the artillery could be used to precede infantry advances with a creeping barrage, or engage in counter-battery duels to try to destroy the enemy's guns. Artillery mainly fired fragmentation, high-explosive, shrapnel or, later in the war, gas shells. The British experimented with firing thermite incendiary shells, to set trees and ruins alight. However, all armies experienced shell shortages during the first year or two of World War I, due to underestimating their usage in intensive combat. This knowledge had been gained by the combatant nations in the Russo-Japanese War, when daily artillery fire consumed ten times more than daily factory output, but had not been applied.