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W. Britain
The British Army sent to mainland Europe in 1914 was referred to as the British Expeditionary Force or BEF.
The BEF was made up of 6 divisions of British regular army and reserves. By the end of 1914 the old regular British Army had suffered massive casualties.
Recruitment of replacements, creation of new division and the expansion of the Territorial Force rebuilt the army on the Western Front with over 5,399,563 men serving in Europe. The maximum strength being 2,046,904 men at any one time.
The casualty rate was often 10% or more on in the trenches. Poor sanitary conditions in addition to disease contributed to this. Poor conditions and the stress of combat broke many men.