 The British Army during World War I fought the largest and most costly war in its long history . Unlike the French and German Armies , its units were made up exclusively of volunteers — as opposed to conscripts — at the beginning of the conflict . Furthermore , the British Army was considerably smaller than its French and German counterparts . During World War I , the British Army grew to over 5 million men — 10 times the size of the French Army and 12 times the size of the German Army .
 everybody who was fit and willing was called upon to serve, regardless of social class. This was a radical departure from the traditional role of