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Although 2,466,719 volunteers had joined the army by the end of 1915, in January 1916 conscription had to be introduced. In all, 5,704,000 men served in the British army during the First World War, split roughly equally between volunteers and conscripts.[137] The army of the First World War was larger by far than any other army raised by Britain, before or since.
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Gary D. Sheffield (Forgotten Victory: The First World War: Myths and Realities)