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.
World War I
After the First Battle of the Marne (5–12 September 1914), Allied and German forces unsuccessfully tried to outflank each other, a series of manoeuvres later known as the "Race to the Sea". By the end of 1914, the opposing forces were left confronting each other along an uninterrupted line of entrenched positions from Alsace to Belgium's North Sea coast. Since the Germans were able to choose where to stand, they normally had the advantage of the high ground; in addition, their trenches tended to be better built, since Anglo-French trenches were initially intended as "temporary," preparatory to breaking the German defences.