British infantry brigades of the First World War

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During the First World War, 259 infantry brigades were raised by the British Army, two by the Royal Navy, and one from the Royal Marines. Of these brigades, fifty-three were held in reserve or only used for training, while another nine only served in British India.

The pre war regular army only had eighteen infantry brigades, with another forty-five serving with the reserve Territorial Force (TF). Once war was declared, the regular army was expanded first by volunteers and then conscripts for what became known as Kitchener's Army. At the same time, volunteers for the TF formed second line formations.

Three infantry brigades served with a division, mostly the same one throughout the war, but some did serve for short periods with another division. At the start of the war, four infantry battalions along with a small headquarters formed a brigade; but, by 1918, with the number of casualties mounting, the brigade was reduced to three battalions. During the same time, the firepower of a brigade was increased by the assignment of more machine guns. Eventually, as the war progressed, a brigade had its own machine gun company and a trench mortar battery assigned.