After Operation Uranus, the counteroffensive of the Battle of Stalingrad, it became painfully obvious that the USSR needed mobile heavy guns in order to destroy German fortifications and heavy tanks. The Red Army did have the KV-2 heavy tank for this duty at the time, but mass production of the KV-2 had ceased when the Kirov Plant was moved to Chelyabinsk. The new anti-fortification vehicle had to be faster than the KV-2, with thicker armor, and mount the powerful and more accurate ML-20 howitzer on a less expensive platform. The gun was too long and its recoil too powerful for it to be mounted in a turret, so a casemate structure was opted for. This would result in the soon-to-be-famous SU-152 'Zveroboy'.