索姆河
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.