All three ships were mobilized as part of the 5th Battle Squadron at Britain's entry into the First World War in August 1914. They were initially based in the English Channel, and they helped to escort the British Expeditionary Force and later the Portsmouth Marine Battalion across the Channel in August. In October "Implacable" was temporarily detached to guard against a rumoured German cruiser attack. Shortly after midnight on 1 January 1915, "Formidable" was torpedoed and sunk by the German U-boat off Portland Bill while on patrol in the Channel. Most of the ship's crew—35 officers and 512 men—were killed in the sinking. The following month, "Irresistible" was sent to the eastern Mediterranean Sea to join the Dardanelles Campaign against the Ottoman Empire. She took part in repeated, failed attacks on the Ottoman defences guarding the entrance to the Dardanelles over the course of February and March. During a major Anglo-French attack on the coastal fortifications on 18 March, "Irresistible" struck a naval mine and sank, though most of her crew was evacuated; around 150 men were killed in her final battle.