Mobilisation began at the onset of the war, in August 1914, at which time the 1st King's was based at Aldershot. Under command of Lieutenant-Colonel W.S. Bannatyne, the 1st King's boarded the SS "Irrawaddy" at Southampton. The battalion landed at Le Havre on 13 August with the 6th Brigade, 2nd Division, one of the original components of the British Expeditionary Force (BEF). The BEF first engaged the German Army at Mons, Belgium, after which it went into a retreat that was sustained until 5 September, when the Allies resolved to stand at the Marne, a river east of Paris. Having acted as a rearguard to the 2nd Division, the 1st King's and its brigade prevented a German force cutting off the 4th (Guards) Brigade, forming the rearguard at Villers-Cotteréts, and 70th Battery, Royal Field Artillery. The brigade extricated the guns, earning it praise from the 2nd Division's commanding officer, Major-General Monro.