On 29 July 1914, "Reşadiye" was seized under the orders of First Lord of the Admiralty Winston Churchill, and on the 31st it entered the Royal Navy as HMS "Erin". She joined the Grand Fleet's 4th Battle Squadron on 5 September of the same year and participated in the fleet's early war sorties and drills, most importantly in response to the German attack on Scarborough, Hartlepool and Whitby. At some point from September to December 1915, "Erin" was transferred to the 2nd Battle Squadron. She participated in the Battle of Jutland, but was hindered by poor visibility and was the only British capital ship to not fire her main battery guns and only fired six 6-inch shells. After Jutland, "Erin" returned to active participation in the Grand Fleet's North Sea operations, sortieing against High Seas Fleet raids on merchant convoys and at the end of war was present for the surrender of the High Seas Fleet at Rosyth, Scotland on 21 November 1918. On 1 May 1919, "Erin" was assigned to the Home Fleet's 3rd Battle Squadron, but entered the reserves at Portland Harbour in October at the Nore. From July to August 1920 "Erin" underwent a refit as a gunnery practice ship, but came to be in violation of the Washington Naval Treaty of 1922 and was sold for scrap on 19 December 1922.