The 125th Brigade was an infantry brigade formation of the British Army that saw active service during both the First and Second World Wars. It was assigned to the 42nd Division and served in the Middle East and later in the trenches of the Western Front in the First World War. In the Second World War the brigade, now redesignated 125th Infantry Brigade, fought in Belgium and France before being evacuated at Dunkirk and was then converted into 10th Armoured Brigade.

The 42nd Infantry Division was an infantry division of the British Army. The division was raised in 1908 as part of the Territorial Force (TF), originally as the East Lancashire Division, and was redesignated as the 42nd Division on 25 May 1915. It was the first TF division to be sent overseas during the First World War. The division fought at Gallipoli, in the Sinai desert and on the Western Front in France and Belgium. Disbanded after the war, it was reformed in the Territorial Army (TA), in the Second World War it served as the 42nd Infantry Division with the British Expeditionary Force (BEF) and fought in Belgium and France before being evacuated at Dunkirk. The division was later reformed in the United Kingdom and, in November 1941, was converted into the 42nd Armoured Division, which was disbanded in October 1943 without serving overseas. A 2nd Line duplicate formation, the 66th Infantry Division, was created when the Territorials were doubled in both world wars.