The German Army first used Panzer I light tanks, along with the Panzer II, but the mainstays were the medium Panzer IIIs and Panzer IVs which were released in 1937. The IV became the backbone of Germany's panzer force and the power behind the blitzkrieg. During the invasion of Russia in 1941, the Germans encountered the famous and technologically advanced Soviet T-34 tanks. This led Germany to develop the Panther or Panzer V in response. Its 75 mm gun could penetrate the new Soviet tanks. Germany also developed the heavy Tiger I, released in 1942. The Tiger could defeat any Allied tank and was soon joined by the Tiger II, also known as King Tiger, but too few were produced to impact the war in any discernible way.