Combat experience in the Spanish Civil War (1936–1939), the Battles of Khalkhin Gol (1939) and the Winter War (1939–1940) showed the Soviet military that light tanks (such as the T-26) were too lightly armoured and that multi-turreted tanks (such as the T-35) were inferior to single turret tanks which guided the switch to the later vital T-34 medium and KV-1 heavy tanks.