The Josif Stalin (IS) tanks (also known as the JS, ИС, or Josif Stalin tanks), were a series of heavy tanks developed by the Soviet Union during World War II and first used in the Kursk area in September 1943.
The heavy tank was designed with thick armour to counter the German 88 mm guns, and carried a main gun that was capable of defeating the German Tiger and Panther tanks. It was mainly a breakthrough tank, firing a heavy high-explosive shell that was useful against entrenchments and bunkers. The IS-2 was put into service in April 1944, and was used as part of the armoured spearhead into Berlin by the Red Army in the final stage of the war. Ingame we have five IS variants: IS-1, IS-100, IS-2, IS-3 and IS-3/D-30.
The KV series of heavy tanks were criticised by their crews for their poor mobility and lack of any heavier armament than the T-34 medium tank. In 1942 this problem was partially addressed by the lighter, faster KV-1S tank. The KV series was much more expensive than the T-34, and lacked the greater combat performance. The heavy tank program was nearly cancelled by Stalin in 1943, however, the German employment of substantial numbers of Panther and Tiger tanks at the Battle of Kursk in the summer of 1943 changed Soviet priorities. In response, the Soviet tank industry created the stopgap KV-85, and embarked on the KV-13 design program to create a tank with more advanced armour layout and a more powerful main gun. The IS-85 (Object 237) prototype was initially accepted for production as the IS-1 heavy tank.