The Iosif Stalin tank (or IS tank, named after the Soviet leader Joseph Stalin), was a heavy tank developed by the Soviet Union during World War II. The tanks in the series are also sometimes called JS or ИС tanks.
The heavy tank was designed with thick armour to counter the German 88 mm guns, and sported a main gun that was capable of defeating the new 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 a spearhead in the Battle for Berlin by the Red Army in the final stage of the war.
The KV series of Soviet heavy tanks was criticized by their crews for their low 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 remained much more expensive than the T-34, without having greater combat performance. The heavy tank program was nearly cancelled by Stalin in 1943. However, the German employment of large 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. Because Marshal Kliment Voroshilov had fallen out of political favour, the new heavy tank series was named Iosif Stalin tank.
The IS-85 prototype was accepted for production as the IS-1 heavy tank, but discussions continued concerning the gun the tank should carry. Two candidate weapons were the A-19 122 mm gun and the BS-3 100mm gun. The 100mm gun (later adopted on the SU-100 tank destroyer and the T-55 tank) had superior armour penetration (185 mm compared to 160 mm), but a less useful high explosive round than the 122mm gun. Also, the 100mm gun was a relatively new weapon in short supply. Excess production capacity existed for the 122mm gun and its ammunition. Proving-ground tests showed that the A-19 could penetrate clear through the front and rear armour of the German Panther tank (Zaloga 1984:172). The IS-122 prototype replaced the IS-85, and began mass production as the IS-2. The 85 mm guns could be reserved for the new T-34/85 medium tank, and the few IS-1s built were mostly rearmed before leaving the factory, and issued as IS-2s.
The main production model was the IS-2, with the powerful A-19. It was slightly lighter and faster than the heaviest KV model 1942 tank, with thicker front armour and a much-improved turret design. The tank could carry thicker armour than the KV series, while remaining lighter, due to the better layout of the armour envelope. The KV's armour was less well-shaped and featured heavy armour even on the rear, while the IS series concentrated its armour up front. The IS-2 weighed about the same as a German Panther and was lighter than the German heavy tank Tiger series. It was slightly lower than both.
The A-19 122mm gun had very good armour penetration, delivering 3.5 times the kinetic energy of the older 76.2mm gun, and when it didn't penetrate, could often knock a turret right off a tank with the combination of the impact and explosive filler. Its very large high explosive shells were very effective against bunkers, infantry and antitank guns. The main disadvantage of the gun was its huge, two-part ammunition, which were difficult to manhandle, slow to reload (the rate of fire was only about two rounds per minute), and only allowed 28 rounds to be carried. (Zaloga 1984:175)
While the design was excellent for its time, Western observers criticized the Soviets for the coarse finish often applied to the tanks. The Soviets responded that it was warranted considering the need for wartime expediency and the typically low battlefield life of their tanks. (Perrett 1987:20)
Early IS-2s can be identified by the 'stepped' front hull casting with its small, opening driver's visor. The early tanks lacked gun tube travel locks or antiaircraft machine guns, and had narrow mantlets.
Later improved IS-2s (the model 1944), had a faster-loading version of the gun, the D25-T with a double-baffle muzzle brake and better fire-control. It also featured a simpler hull front without a "step" in it (using a flat, sloping glacis armour plate). Some sources called it IS-2m, but it is not to be confused with the official Soviet designation IS-2M for a 1950s modernization. Other minor upgrades included the addition of a travel lock on the hull rear, wider mantlet, and, on very late models, an antiaircraft machine gun.
In late 1944 the design was upgraded to the IS-3. This tank had improved armour layout, and a hemispherical cast turret (resembling an overturned "soup bowl") which was to be the hallmark of post-war Soviet tanks. While this low, hemispherical turret may have made the IS-3 a smaller target, it also imposed severe penalties inside the tank by significantly diminishing the working headroom, especially for the loader (Soviet tanks in general are characterized by uncomfortably small interior space compared to Western tanks). The low turret also limited the maximum depression of the main gun, since the gun breech had little room inside the turret to pivot on its vertical axis. As a result, the IS-3 was unable to use cover provided by the reverse sides of hills and embankments, and it had to expose itself by driving over the crest to fire at opponents. (Perrett 1987:21) Western tanks were better suited to fighting in the hull-down position. The IS-3s pointed prow earned it the nickname Shchuka (Pike) from its crews. Its front hull armour was thicker than the IS-2's (200 mm versus 120 mm), although it weighed slightly less and stood 30 cm lower.
The IS-3 came too late to see action in World War II. Though some older sources claim that the tank saw action at the end of the war in Europe, there are no official reports to confirm this. It is now generally accepted that the tank saw no action against the Germans, although one regiment may have been deployed against the Japanese in Manchuria.
In 1952, a further development was put into production, the IS-10. Due of the political climate in the wake of Stalin's 1953 death, it was renamed T-10.
In the mid-1950s, the remaining IS-2 tanks (mostly model 1944 variants) were upgraded to keep them battle-worthy. This upgrade produced the IS-2M, which introduced fittings such as external fuel tanks on the rear hull (the basic IS-2 had these only on the hull sides), stowage bins on both sides of the hull, and protective skirting along the top edges of the tracks. IS-3 was also slightly modernized as IS-3M.
One of the IS-2's most notable engagements took place during the fighting in August 1944 to establish a bridgehead across the river Vistula around the town of Sandomierz. This was the first time the IS-2 had come up against the Tiger II. During the engagement on August 13, the 71st Independent Heavy Tank Regiment's eleven IS-2s blocked an attack by fourteen Tiger IIs of the 105th Heavy Panzer Regiment. An engagement at about 700 metres (770 yd) coupled with skilled tactical handling saw four Tiger IIs destroyed for the loss of three IS-2s and seven damaged.
The IS-3 first appeared to Western observers at the Allied Victory Parade in Berlin in September 1945. By most accounts, these tanks overawed the Western powers, who responded with heavy tank designs of their own in the 1950s.
By the 1950s, the emergence of the main battle tank concept rendered heavy tanks obsolete. The main battle tank concept combined the mobility of the medium tank with the armour and firepower of the heavy tank. In the late 1960s, the remaining Soviet heavy tanks were transferred to Red Army reserve service and storage. The IS-2 Model 1944 remained in active service much longer in the armies of Cuba, China and North Korea. A regiment of Chinese IS-2s was available for use in the Korean War, but saw no service there. Some Soviet IS-3s were dug in as fixed pillboxes along the Soviet-Chinese border. The IS-3 was used in the 1956 Soviet invasion of Hungary and the Prague Spring in 1968. It was supplied to Egypt, seeing action in the Six Day War against IDF M48 Pattons, but by this time the concept of the heavy tank was showing its deficiencies.
After the Korean War, China attempted to reverse-engineer the IS-2/IS-3 as Type 122 medium tank. The project was cancelled in favour of the Type 59, a copy of the Soviet T-54A.