The IS-2, named for Joseph Stalin, grew out of the failing KV-1 program. By early 1943 the KV-1 was slow, prone to transmission failure and armed no better than the T-34, and Soviet command considered ending heavy tank production and closing Joseph Kotin’s design bureau. The capture of a German Tiger I in January 1943 reversed that. Stopgaps followed: the lighter KV-1S, then the KV-85, which needed welded fillets to widen the hull for a turret carrying the 85mm D-5T gun, with only 130 built before the IS-1 (Object 237). For the IS-2, planners chose the 122mm D-25, adapted from the A-19 artillery piece, over the 100mm D-10 even though the 100mm penetrated German armor better, because 122mm production capacity and ammunition stocks already existed and its 25-kilogram high-explosive shell could wreck a tank without piercing it.

The big gun used separate-loading ammunition, so early tanks fired one to one and a half rounds a minute until a semi-automatic drop breech raised that to three or four. Only 28 rounds fit inside, typically 20 high-explosive and 8 armor-piercing. The hull relied on cast armor, cheaper and easier to angle but uneven in thickness, and Soviet steel substituted manganese for scarce nickel and grew brittle. German industry ran short of manganese by summer 1944, leaving Panther weld seams that could crack from a ricochet at 2,500 meters. On August 11, 1944 at Oględów, IS-2s waiting 800 meters from roads that Tiger IIs of the 16th Panzer Division could not leave in sandy ground destroyed three, and three more were found abandoned and sent to Kubinka.

• The 122mm high-explosive round carried a 3.8-kilogram TNT charge at 800 meters per second and caused lethal spalling inside enemy hulls.
• Late-1944 hulls replaced the stepped front and opening driver’s visor with a single 120mm cast nose angled at 60 degrees.
• One Guards senior lieutenant’s IS-2 logged 955 kilometers and 205 engine hours by August 1944 against a 150-hour warranty.
• A German report from October 1944 found a Tiger I had to close to 100 to 300 meters to pierce the IS-2’s front, while the IS-2 could pierce the Tiger from 500 to 1,500 meters.
• After the war the IS-2 went to China, Cuba and North Korea, was modernized as the IS-2M in the 1950s, and moved to reserve storage by the late 1960s.