Prior to the invasion of the Soviet Union during World War II, the German armed forces were not aware of two newly developed Soviet tanks, the T-34 and the KV. As a result, they were surprised when they met them in combat for the first time in June 1941. The Germans' standard anti-tank weapons were found to be ineffective against these new Soviet vehicles. This experience prompted a notable leap in tank development in Nazi Germany, mainly in an effort to counter these new threats.