At the start of the First World War, she was involved in the pursuit of the German warships "Goeben" and "Breslau", but Rear-Admiral Ernest Troubridge decided not to engage "Goeben" due to the latter's more powerful guns, heavier armour and faster speed. She then blockaded the German ships inside the Dardanelles until the ship was ordered on 10 September to the South Atlantic to take part in the hunt for Admiral Graf von Spee's East Asia Squadron. The order was cancelled on 14 September when it became clear that the German squadron was still in the Eastern Pacific and "Defence" returned to the Dardanelles. The Admiralty again ordered the ship to the South Atlantic in October to join Rear-Admiral Christopher Cradock's squadron searching for the German ships. "Defence", however, had only reached Montevideo, Uruguay by 3 November 1914 when she received word that most of Admiral Cradock's squadron had been destroyed two days previously at the Battle of Coronel. The ship rendezvoused with the battlecruisers and later that month and transferred her long-range radio equipment to "Invincible" before sailing to South Africa to escort a troop convoy to Great Britain. "Defence" departed Table Bay, Cape Town on 8 December and rejoined the 1st Cruiser Squadron of the Grand Fleet as its flagship upon her arrival.