On the outbreak of the First World War in August 1914, the Royal Navy mobilised to meet the German High Seas Fleet. On 7 August, the 6th Battle Squadron was dissolved, the more modern s being transferred to the Grand Fleet; "Vengeance" was assigned to the 8th Battle Squadron, Channel Fleet. The 8th Squadron was tasked with patrol duties in the English Channel and Atlantic, though "Vengeance" was quickly transferred to the 7th Battle Squadron on 15 August 1914 to relieve the battleship as the squadron flagship. Shortly thereafter, half of the 7th Squadron battleships were dispersed to strengthen the detached cruiser squadrons patrolling for German commerce raiders, leaving only "Vengeance", "Prince George", , and "Goliath" and the protected cruiser . She covered the landing of the Plymouth Marine Battalion at Ostend, Belgium, on 25 August 1914. For this operation, she and the other five ships of the squadron, along with six destroyers, escorted the troopships; at the same time, elements of the Grand Fleet attacked the German patrol line off Heligoland to occupy the High Seas Fleet.