The IS tank (or Iosif Stalin Tank) was a series of heavy tanks developed as a successor to the KV-series by the Soviet Union during World War II. It was named after Soviet leader Joseph Stalin.
The heavy tank was designed with a very thick armor to counter the German 88 mm guns, and carried a very powerful main gun that was capable of defeating the German Tiger I and Panther tanks in one shot.
Because of its low mobility and speed it was mainly a breakthrough tank, firing a heavy high-explosive shell that was useful against heavy fortifications like entrenchments and bunkers.
