1914 British Expeditionary Force (World War I)

the race sea, allied front line , movements shown in red, german front line , movements shown in blue

the british army first engaged german army in battle of mons on 23 august 1914, part of greater battle of frontiers. massed rifle fire of professional british soldiers inflicted heavy casualties on germans attacked en masse on terrain devoid of cover. british held german advance until evening when began retiring second defensive line in retreat mons during engaged in battle of le cateau. following british defeat @ le cateau, bef retreated unmolested germans further 5 days, retreating full 400 km in thirteen days.

the allied retreat ended @ river marne, entente forces prepared make stand defend paris. led first battle of marne, fought 5 10 september 1914. battle prove major turning point of war: denied germans victory. 13 september first battle of aisne took place, both sides starting dig trenches. three-week period following development of trench warfare each side gave frontal assaults , began trying encircle each other s flank. period became known race sea: germans aimed turn allied left flank, , allies sought turn german right flank.

the front line in 1916, british gains during battle of somme shaded blue.

by end of first battle of ypres both sides started dig in , trench warfare replaced manoeuvre warfare had featured during race sea. continuous trench lines of western front stretched 400 miles (640 km) north sea swiss frontier. british army held small portion of 400 mile front: north of pre-war belgium border river somme in france, varying in length 20 miles (32 km) in 1914, on 120 miles (190 km) in 1918, in area commonly known flanders.