The 91st Brigade was an infantry formation of the British Army during World War I. It was raised as part of 'Kitchener's Army' and was assigned to the 30th Division. After the original formation was converted into a reserve brigade, the number was transferred to a brigade of 'Manchester Pals'. The brigade landed in France at the end of 1915 and was transferred to the Regular 7th Division. It saw action at the Somme, Arras, and Ypres before being sent to the Italian Front, where it took part in the final Battle of Vittorio Veneto. The brigade's number was briefly revived during the 1950s.

Operations on the Ancre took place from 11 January – 13 March 1917, between the British Fifth Army and the German 1st Army, on the Somme front during the First World War. After the Battle of the Ancre, British attacks on the Somme front stopped for the winter. Until early January 1917, both sides were reduced to surviving the rain, snow, fog, mud fields, waterlogged trenches and shell-holes. British preparations for the Battle of Arras, due in the spring of 1917, continued.