The Kliment Voroshilov tanks were a series of Soviet heavy tanks named after the Soviet defense commissar and politician Kliment Voroshilov and used by the Red Army during World War II. The KV series were known for their extremely heavy armour protection during the early part of the war, especially during the first year of the German invasion of the Soviet Union. They were almost completely immune to the 3.7 cm KK 36 and howitzer-like, short barreled 7.5 cm KK 37 guns mounted respectively on the early Panzer III and Panzer IV tanks fielded by the invading German forces. Until better guns were developed by the Germans it was often the case that the only way to defeat a KV was with a point-blank shot to the rear.