The Kliment Voroshilov-1 tank was a heavy tank developed by the Soviet Union in 1939. The tanks were resistant to anti-tank weaponry during the Winter War with Finland, but they were prone to breakdowns, and they were phased out of service in 1943 as the KV-2 model replaced the KV-1. 5,219 KV-1s were built between 1939 and 1943.