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.
Anti-tank rifle
The first tanks, beginning with the British Mark I launched against the German trenches in World War I, were nearly impregnable to ordinary rifle fire. Most armoured cars were similarly protected, but troops rarely faced armoured cars, as they could not navigate the landscape of trench warfare very well. Though tanks and armoured cars were vulnerable to artillery, mortars, and grenades, infantry was at a significant disadvantage when facing armoured fighting vehicles since they had no effective direct fire weapon, with the exception of the ubiquitous trench mortar, improvised on the spot. In the direct fire mode, this weapon was manhandled by German infantry over the front of a trench wall and fired at low angles by eye at approaching enemy vehicles. Though somewhat effective, these actions were obviously very hazardous to any desperate mortar crew as their exposure could attract enemy fire.