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.
Henri Mordacq
In August 1914, Mordacq was chief of staff of general Archinard's First Reserve Corps Group in the Eastern Army. When the Germans breached the French lines through Belgium, he asked for a frontline command and became commander of the famous 159th Regiment of Alpine Infantry. He immediately took part in the Battle of the Frontiers and his regiment participated in the ending of the German advance in the Vosges before the First Battle of the Marne led to the Race to the Sea. He then took command of the defence of Arras, the "key to the sea" and again halted the German advance there, which led to the stabilization of the front and the beginning of trench warfare.