IS-2 Soviet heavy tank, model 1944, Poland The IS Tank (IS in Cyrillic "ИС", meaning the Joseph Stalin or Iosif Stalin in Cyrillic "Ио́сиф Ста́лин") was a series of heavy tanks developed as a successor to the KV-series by the Soviet Union during World War II and was named after Soviet leader Joseph Stalin. The heavy tank was designed with thick armour to counter German 88 mm guns and carried a main gun capable of defeating Tiger and Panther tanks. It was mainly a breakthrough tank, firing a heav