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.
Barrage (artillery)
Artillery usually fired over open sights at visible targets, until the Second Boer War when indirect fire started to be used. The largest unit accustomed to firing at a single target was the brigade (i.e. an artillery battalion), normally 18 guns. Trench warfare led to the necessity for indirect firing through the use of observers, more sophisticated artillery fire plans and an increasingly scientific approach to gunnery. Gunners had to use increasingly complicated calculations to lay the guns. Individual guns were aimed so that their fall of shot was co-ordinated with others to form a pattern; in the case of a barrage, the pattern was a line. The term “barrage” was first used in World War I in English in the orders for the Battle of Neuve Chapelle in 1915.