Promoted to brevet major on 1 July 1929, made permanent on 24 October 1930, from 31 October 1931 until 4 December 1933 Barker was brigade major of the 8th Infantry Brigade, part of the 3rd Infantry Division, then serving as part of Southern Command. Made a brevet lieutenant colonel on 1 July 1934, in June 1936 Barker succeeded Lieutenant Colonel Ralph Eastwood, who had been one of his instructors at the Staff College, as Commanding Officer (CO) of the 2nd Battalion, KRRC, and was promoted to lieutenant colonel on 13 July. The battalion was then stationed in Palestine on internal security duties during the Palestinian Arab Revolt of 1936–39 as part of Brigadier Pierse Mackesy's 2nd Infantry Brigade. In 1937 the battalion was converted into a motorised infantry role and, later in the year, was sent to England, where it became part of the Mobile Division (later the 1st Armoured Division), the forerunner of the armoured divisions of the future, then commanded by Major General Alan Brooke.