Eight million men in total fought in the British Army during the First World War. More than half of them (5 million) served in France and Flanders on the Western Front.  These men came from the U.K. as well as Canada, Australia, New Zealand, South Africa, India and even the West Indies. Another 8 million soldiers fought for France.  The United States sent 4.7 million to war with Germany in the last two years of the war  and as many as 11 million Germans fought in France and Belgium as well between 1914 and 1918.
The Forgotten Armies of the Western Front - 1914 to 1918
Mr. Mallet often writes on little known facts. I enjoy that!