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.
Race to the Sea
Over the winter lull, the French army established the theoretical basis of offensive trench warfare, originating many of the methods which became standard for the rest of the war. Infiltration tactics, in which dispersed formations of infantry were followed by (trench cleaners), to capture by-passed strong points were promulgated. Artillery observation from aircraft and creeping barrages, were first used systematically in the Second Battle of Artois from 9 May to 18 June 1915. Falkenhayn issued memoranda on 1915, to govern defensive battle on the Western Front, in which the existing front line was to be fortified and to be held indefinitely with small numbers of troops, to enable more divisions to be sent to the Eastern Front. New defences were to be built behind the front line to contain a breakthrough until the position was restored by counter-attacks. The began the huge task of building field fortifications, which were not complete until the autumn of 1915.