In 1939 and 1940 Kempf led the 6th Panzer Division in the Battle of France. He was awarded the Knight's Cross of the Iron Cross on 3 June 1940 for his role in the campaign, and was promoted to "Generalleutnant" on 1 August 1940. On 6 January 1941, he was ordered to form XXXXVIII Army Corps (motorized), and became its commander, along with a promotion to "General der Panzertruppe", on 1 April 1941. With this corps Kempf took part in Operation Barbarossa, the invasion of the Soviet Union, starting on 22 June 1941, as part of Panzer Group 1 of Army Group South, where the corps took part in the Battle of Uman and Battle of Kiev (1941), and pushed as far as Kursk.