On 1 January 1938 Stumpff was given command of the 3rd Panzer Brigade and on 1 March 1939 he was promoted to Generalmajor and led the brigade during the invasion of Poland. On 7 October 1939 he was appointed commander of the 3rd Panzer Division and in November 1940 he was given command of the new 20th Panzer Division and promoted to Generalleutnant on 1 February 1941. He led the division on the Eastern Front and on 29 September 1941 he was awarded the Knight's Cross of the Iron Cross, but he was moved to the Führerreserve shortly afterwards.