Trench warfare
Trench warfare occurred when a revolution in firepower was not matched by similar advances in mobility, resulting in a grueling form of warfare in which the defender held the advantage. On the Western Front in 1914–1918, both sides constructed elaborate trench, underground, and dugout systems opposing each other along a front, protected from assault by barbed wire, mines, camouflaged trapping pits, and other obstacles. The area between opposing trench lines (known as "no man's land") was fully exposed to artillery fire from both sides. Attacks, even if successful, often sustained severe casualties.
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.