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.
British Expeditionary Force (World War I)
By the end of First Battle of Ypres both sides started to dig in and trench warfare replaced the manoeuvre warfare that had featured during the Race to the Sea. The continuous trench lines of the Western Front now stretched from the North Sea to the Swiss frontier. The British Army held a small portion of this 400 mile front: from just north of the pre-war Belgium border to the River Somme in France, varying in length from in 1914, to over in 1918, in the area commonly known as Flanders.