In January 1915, "Cornwallis" was sent to the Mediterranean to take part in the Dardanelles campaign against the Ottoman Empire. She fired the first shots of the campaign on 19 February during a bombardment of Ottoman coastal defences. Over the following two months, she participated in numerous attacks on the forts that failed to destroy them, leading to the decision that a major ground attack would be necessary to neutralise the defences. "Cornwallis" supported the Landing at Cape Helles on 25 April and shelled Ottoman troops over the following month as the Allied soldiers sought to push further inland. She thereafter served with the Suez Canal Patrol and briefly on the East Indies Station until March 1916, when she returned to the Mediterranean. While on patrol off Malta on 9 January 1917, she was torpedoed and sunk by the German U-boat .