On the Soviet side, based upon lessons from the Spanish Civil War, the Red Army decided to develop a heavy -breakthrough- tank to smash enemy infantry defenses. This resulted in the KV-1 and KV-2 tanks, introduced in 1939. At the start of Operation Barbarossa, both these tanks were virtually invulnerable to the weapons of the Panzerj-r and demonstrated their ability to overrun German infantry on several occasions. This advantage gave the Red Army a window of opportunity between the fall of 1941 and the spring of 1942 to use their heavy tanks to repel the German invasion in a series of desperate counteroffensives. Yet the window of Soviet advantage was a narrow one and the duel between the Soviet KV heavy tanks and German Panzerj-r had a major impact upon the struggle for the strategic initiative in 1941-42.
