The IS Tank was a series of heavy tanks developed as a successor to the KV-series by the Soviet Union during World War II. The IS acronym is the anglicized initialism of Joseph Stalin (Ио́сиф Ста́лин, Iosif Stalin). The heavy tanks were designed with thick armor to counter German 88 mm guns and carried a main gun capable of defeating Panzer IV tanks. They were mainly designed as breakthrough tanks, firing a heavy high-explosive shell that was useful against entrenchments and bunkers. The IS-2 went into service in April 1944 and was used as a spearhead by the Red Army in the final stage of the Battle of Berlin. The IS-3 served on the Chinese-Soviet border, the Hungarian Revolution, the Prague Spring and on both sides of the Six-Day War. The series eventually culminated in the T-10 heavy tan