The ISU-152 was an extension of the Su-152 self-propelled heavy assault gun vehicle (detailed elsewhere on this site) developed by the Soviet Union in late 1942 for service in World War 2. After recovering from the initial shock of the German invasion of 1941, the Soviet Army realized it required a heavy assault weapon to dislodge enemy positions at range - particularly in urban settings. This led to a period of quick study and evaluation that resulted in the KV-1 Heavy Tank chassis being reworked to accept a fixed, boxy armored superstructure to house the 155m ML-20 gun-howitzer. The result was the Su-152.
While successful for its time in the latter half of 1943, there was still room for improvement and this ultimately arrived in the form of the "ISU-152".
Unlike the earlier Su-152, the ISU-152 used the chassis and running gear of the newer IS (Josef Stalin) heavy tank series and were powered by a Kharkiv Model V-21S diesel-fueled engine of 520 horsepower. The crew number remained four or five and an optional 12.7mm DShK Heavy Machine Gun (HMG) could still be installed over the right side of the hull superstructure roof. The driver sat at front-left with the gunner and ammunition loader seated in line directly behind. The vehicle commander and breech operator were positioned to the right side of the hull superstructure. A pair of submachine guns (PPSh) were carried by the crew for extreme-close-in defense of the vehicle. A torsion bar suspension system provided the vehicle with some cross-country capability. The primary armament became the ML-20S gun-howitzer. Up to 18 x 155mm projectiles could be carried.
The ISU-152 was essentially a modernized version of the preceding Su-152 vehicle and its pilot form was tested at-speed from the period spanning September until November 1943. With revisions forced about due to issues that arose during this evaluation, the ISU-152 finally emerged as a (somewhat) combat-ready platform in short order. In December, serial production had already begun in time for the fighting of 1944 and, before the end of the war, over 1,800 ISU-152s would be produced and this work would continue until 1947 where total production would yield some 3,242 total units. By the end of its useful service life, various other modern systems were introduced to the series to help keep the vehicle as viable as possible for the modern battlefield. This also helped to sell the vehicle on the export market.
Beyond the base ISU-152 production model was the ISU-152-2, pilot vehicle of 1944 used in an attempt to improve on the design's inherent firepower. ISU-152K was a development of 1956 and a modernization of the aging wartime models while ISU-152M followed in 1959 as the final modernized form. "Object 704" was a related project of 1945 used to showcase the potential mix of IS-2/IS-03 heavy tank components in a new ISU-152 design - it was not adopted and the war drew to a close that year anyhow.
Operators of the product (beyond the Soviet Union) went on to include China, Czechoslovakia, Egypt, Iraq, North Korea, Poland, Romania and Yugoslavia. Finland captured several examples during the "Continuation War" in 1944 with the Soviet Union. Iraq fielded the weapon in its war against neighboring Iran during the Iran-Iraq War of 1980-1988 - some examples were still in service during the 1991 Gulf War that followed. North Korea utilized the tank in the Korean War of 1950-1953 and Egypt took the ISU-152 into battle against neighboring Israel during the various Egyptian-Israeli wars that began in 1948. The Romanian Army designated the ISU-152 as the "T-152".