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)
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.