Promotion in the interwar army was slow and Crocker's advancement was evidenced by a succession of brevet ranks: brevet major on 1 January 1935, brevet lieutenant colonel on 1 July 1936 and brevet colonel on 1 February 1938. However, his permanent rank caught up when he was promoted to full colonel on 6 August 1938 (with seniority backdated to 1 February 1938). By the time the Second World War began in September 1939 he was a GSO1 staff officer in Southern Command.