Grenzschlachten
In August 1914, Mordacq was chief of staff of general Archinard's 1st 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 . He immediately partook in the Battle of the Frontiers and his regiment participated in ending 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 front's stabilization and the start of trench warfare.