In April 1941, Kübler's XXXXIX Mountain Corps was allocated to the 2nd Army for the German-led Axis invasion of Yugoslavia, and assembled near Klagenfurt prior to the commencement of the invasion on 6 April. By this time, Kübler had been promoted to "General der Infanterie". For the Axis attack on the Soviet Union that commenced in 22 June 1941, Kübler's corps was allocated to the 17th Army, itself part of Army Group South. Kübler's rank was redesignated from "General der Infanterie" to "General der Gebirgstruppe". On 19 December 1941, Kübler was appointed commander of the 4th Army. This occurred during the desperate winter defensive fighting when Adolf Hitler would not countenance any withdrawals. Kübler found Hitler's approach "intolerable" and sought relief. On 20 January 1942 he was transferred to the Army "Führerreserve" (senior officer reserve pool).