At the start of World War II, Kübler's division was involved in the invasion of Poland as part of the 14th Army, commencing on 1 September 1939. It fought in the Carpathian Mountains, distinguishing itself during the capture of the Dukla Pass. On 27 October, Kübler was awarded the Knight's Cross of the Iron Cross for his role in the Polish campaign. Then on 1 December 1939 he was promoted to Generalleutnant. During the invasion of France and the Low Countries in May 1940, Kübler's division was not heavily committed, fighting on the Franco-Belgian border and crossing the Maas on 15 May. The division was stationed around Arras after the fall of France, as it had been selected to form part of the invasion force for Operation Sealion, the invasion of the United Kingdom. After that operation was postponed, on 25 October 1940 Kübler was appointed to command the XXXXIX Mountain Corps, which included the 1st Mountain Division. His corps was earmarked for Operation Felix, the planned capture of Gibraltar, but that operation was also called off. Due to the disfigurement caused by the wounds he suffered during World War I, Kübler became known to his troops as Limping Nurmi, a reference to the Finnish long-distance runner Paavo Nurmi.