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)
The Allied retreat finally ended at the River Marne, where Entente forces prepared to make a stand to defend Paris. This led to the First Battle of the Marne, which was fought from 5 to 10 September 1914. This battle would prove to be a major turning point of the war: it denied the Germans an early victory. From 13 September the First Battle of the Aisne took place, with both sides starting to dig trenches. Then for a three-week period following the development of trench warfare each side gave up frontal assaults and began trying to encircle each other's flank. This period became known as the Race to the Sea: the Germans aimed to turn the Allied left flank, and the Allies sought to turn the German right flank.