Soviet Heavy Tank IS-1 (Iosif Stalin - 1)

The need of mightier than KV tank arose because of higher effectivity of German anti-tank defense and awaited appearing of new vehicles Tiger and Panther. In the beginning of December, 1943 the mass production of IS-1 or IS-85 has begun.
Tank was equipped with 85mm half-automatic cannon constructed by F.F.Petrov and its weight was a little more than of KV-1S, but had thicker armour. Its width was also 18cm lesser than of KV-1S due to installation of smaller turning mechanisms constructed by A.I.Blagonravov.
But after the beginning of IS-1 production the question of strengthening heavy tank's armament was not answered. T-34-85 had also 85mm cannon installed. It was not very wise to have medium and heavy tanks with the same armament.

Soviet Heavy Tank IS-2 (Iosif Stalin - 2)

The tank with the new 122mm tank half-automatic cannon also constructed by F.F.Petrov goes to the mass production at 31th December, 1943. IS-2 was born. Thanks to good costruction the size of IS-2 was not increased in comparison with KV, but the speed and manoeuverability were higher. The vehicle was easy to drive and was easy to repair at field conditions. At the 500m range the cannon shell pierced armour up to 140mm thick. IS-2 first battle was at Korsunj-Shevchenskij in February, 1944, where it shows its excellent combat abilities. Before the end of war, IS-2 was modernizated. In the second quarter of 1944, tank received better targeting instruments and larger cannon armour mask. From the middle of 1944 had the same form of hull as T-34's one. Tank was named IS-2M. Soviet authors said that IS-2 was the mightest tank of the Second World War and that the German command ordered its tankists to avoid the open duels with IS-2.