HMS Erin was a dreadnought battleship of the Royal Navy, originally ordered by the Ottoman government from the British Vickers Company. The ship was to have been named Reşadiye when she entered service with the Ottoman Navy. The Reşadiye class was designed to be at least the equal of any other ship afloat or under construction. 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. There is no evidence that the seizure played any part in the Ottoman government declaring war on Britain and the Triple Entente.

By taking on ships being built for foreign navies in British shipyards, a number of British-built 6-inch 50-calibre naval guns found their way into British service in World War I. Their specifications and performance differed from standard Royal Navy 6-inch guns but in British service they fired standard service 100-pound projectiles.