British Army during the First World War

The British Army during World War I fought the largest and most costly war in its long history. Unlike the French and German Armies, the British Army was 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, there were three distinct British Armies. The "first" army was the small volunteer force of 400,000 soldiers, over half of which were posted overseas to garrison the British Empire. This total included the Regular Army and reservists in the Territorial Force (TF). Together, they formed the British Expeditionary Force (BEF), which was formed for service in France and became known as the Old Contemptibles. The 'second' army was Kitchener's Army, formed from the volunteers in 1914–1915 destined to go into action at the Battle of the Somme. The 'third' was formed after the introduction of conscription in January 1916, and by the end of 1918, the British Army had reached its maximum strength of 4,000,000 men and could field over 70 divisions. The vast majority of the British Army fought in the main theatre of war on the Western Front in France and Belgium against the German Empire. Some units were engaged in Italy and Salonika against Austria-Hungary and the Bulgarian Army, while other units fought in the Middle East, Africa and Mesopotamia—mainly against the Ottoman Empire—and one battalion fought alongside the Japanese Army in China during the Siege of Tsingtao.

...
Wikipedia