Recruitment to the British Army during World War I
At the beginning of 1914 the British Army had a reported strength of 710,000 men including reserves, of which around 80,000 were professional soldiers ready for war. By the end of the First World War almost 25 percent of the total male population of the United Kingdom of Great Britain and Ireland had joined up, over five million men.
Source: Wikipedia — Recruitment to the British Army during World War I (CC BY-SA 4.0)