During the First World War, British intelligence solved the United States' diplomatic codes and were reading its diplomatic telegrams transmitted between Washington and US diplomatic outposts throughout Europe. Controversy has emerged over when the British succeeded in solving these codes, with two historians relatively recently having claimed that British intelligence succeeded in doing so from the beginning of the war or soon after. Through a thorough consideration of the available documentation, this piece aims to correct these mistaken claims and to date the completion of the British solving of American codebooks to the middle phase of the war, to between October 1915 and January 1916. It seeks to lay reliable foundations for further work by showing that research into the wartime impact of British signals’ intelligence on Anglo-American relations is necessarily limited to only the middle and later phases of the war.