173rd (3/1st London) Brigade

The 173rd (3/1st London) Brigade was a territorial infantry brigade of the British Army, formed as part of the 58th (2/1st London) Division during the First World War. The designation indicates it was the 173rd Brigade in the order of battle, and specifically the 3rd Line battalion of the 1st London Brigade. These "3rd Line" units were formed in 1915 from men who had volunteered to serve in the Territorial Force but were not immediately deployed overseas. They served as training and reinforcement units for their "1st Line" and "2nd Line" counterparts, which were actively serving in theaters of war.

The Brigade's primary role was to train and equip soldiers for service abroad, replacing casualties and maintaining the strength of the 1st and 2nd London Brigades. It would have been involved in drill, musketry, and tactical exercises to prepare recruits for the realities of trench warfare. While primarily a training unit, the Brigade also played a role in home defense and maintaining order within its designated area. After providing reinforcements and training, 3rd Line units were often disbanded or reorganized as the need for replacement troops diminished towards the later stages of the war. Detailed information regarding the specific battalions composing the brigade and its exact movements and disbandment details would require further historical research and consulting of regimental histories.