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 map
For much of the Great War, trench warfare was almost static, giving rise to the need for large scale maps for attack, defence and artillery use. Initially, British trench maps showed the German trench systems in detail, but only the British Front line. Later in the war, more of the British trenches were shown. The only British maps that showed full detail of both sides were the secret editions, usually marked "Not to be taken beyond Brigade HQ" for fear of their falling into enemy hands.