Formed in late 1918 during the Russian Civil War as the 5th Ural Infantry Division, the division fought on the Eastern Front. In September 1919 it was split up and two of its brigades and its headquarters transferred west. The division was reunited on the Southern Front in January, fighting in the final stages of the campaign there. In May it was transferred to fight in the Polish–Soviet War, and was one of the divisions that fought in the Battle of Radzymin during the larger Battle of Warsaw. After the Red Army was defeated by a Polish counterattack, the division retreated into Belarus. It transferred east again to suppress a peasant revolt in Western Siberia, and remained there until 1929, when it transferred to the Soviet Far East to fight in the Sino-Soviet conflict of that year. After Operation Barbarossa began on June 22, 1941, the division was sent west in September and fought in the Continuation War against Finland in Karelia. From the spring of 1944 it fought in the Arctic sector, initially in the area of Alakurtti. After the withdrawal of Finland from the war, the division was transferred to Hungary in January 1945. It fought in the end of the Budapest Offensive, Operation Spring Awakening, and the Vienna Offensive at the end of the war. Postwar, the 21st was withdrawn to Romania and converted into the 20th Mechanized Division, which was disbanded in 1947.