[1] When the First World War began in August 1914, Reşadiye was nearly complete and was seized at the orders of Winston Churchill, the First Lord of the Admiralty, to keep her in British hands and prevent her from being used by Germany or German allies.

[citation needed] Aside from a minor role in the Battle of Jutland in May 1916 and the inconclusive Action of 19 August the same year, Erin's service during the war generally consisted of routine patrols and training in the North Sea.

The ship carried enough coal and fuel oil for a maximum range of 5,300 nautical miles (9,800 km; 6,100 mi) at a cruising speed of 10 knots (19 km/h; 12 mph).

[2] The ship was armed with a main battery of ten BL 13.5 in (343 mm) Mk VI guns mounted in five twin-gun turrets, designated 'A', 'B', 'Q', 'X' and 'Y' from front to rear.

[8] A pair of directors for the secondary armament were fitted to the legs of the tripod mast in 1916–1917 and another three-inch AA gun was added on the aft superstructure.

[Note 3] She was laid down at the Vickers shipyard in Barrow-in-Furness on 6 December 1911 with yard number 425, but construction was suspended in late 1912 during the Balkan Wars and resumed in May 1913.

[14] Churchill ordered the Royal Navy to detain the ships on 29 July and prevent Ottoman naval personnel from boarding them; two days later, soldiers from the Sherwood Foresters Regiment formally seized them and Reşadiye was renamed Erin, a dative name for Ireland.

[15] Churchill did this on his own initiative to augment the Royal Navy's margin of superiority over the German High Seas Fleet and to prevent them from being acquired by Germany or its allies.

[18] Although there is no evidence that the seizure played any part in the Ottoman government declaring war on Britain and the Triple Entente,[19] historian David Fromkin has speculated that the Turks promised to transfer Sultan Osman I to the Germans in exchange for signing a secret defensive alliance on 1 August.

[21] On 5 September, she joined the Grand Fleet, commanded by Admiral John Jellicoe, at Scapa Flow in Orkney and was assigned to the Fourth Battle Squadron (4th BS).

[11] Erin steamed with the ships of the Grand Fleet as they departed from Loch Ewe in Scotland on 17 September for gunnery practice west of the Orkney Islands the following day.

The Grand Fleet arrived at Scapa Flow on 24 September to refuel before departing the next day for more target practice west of Orkney.

Reports of U-boats in Scapa Flow led Jellicoe to conclude that the defences there were inadequate, and on 16 October he ordered that the bulk of the Grand Fleet be dispersed to Lough Swilly, Ireland.

Jellicoe took the Grand Fleet to sea on 3 November for gunnery training and battle exercises, and the 4th BS returned to Scapa six days later.

On the evening of 22 November, the Grand Fleet conducted another abortive sweep in the southern half of the North Sea; Erin stood with the main body in support of Vice-Admiral David Beatty's 1st Battlecruiser Squadron.

On the night of 25 March, Erin and the rest of the fleet sailed from Scapa Flow to support Beatty's battlecruisers and other light forces raiding the German Zeppelin base at Tondern.

[32] On 21 April, the Grand Fleet conducted a demonstration off Horns Reef to distract the Germans while the Russian Navy re-laid its defensive minefields in the Baltic Sea.

[34] During the Battle of Jutland on 31 May, Beatty's battlecruisers managed to bait Scheer and Hipper into a pursuit as they fell back upon the main body of the Grand Fleet.

The British only learned of the operation after an accident aboard the battlecruiser SMS Moltke forced her to break radio silence and inform the German commander of her condition.

[40] The ship was at Rosyth, Scotland, when the surrendered High Seas Fleet arrived on 21 November and she remained part of the 2nd BS through 1 March 1919.