Battles of Khalkhin Gol
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.