Following the invasion of the Soviet Union in 1941, a significant problem developed when it was discovered that the main armament on the Panzer II and III tanks were inadequate to deal with the newer Soviet T-34 and KV-1 tanks. The main German anti-tank gun, the 37 mm Pak 36, also proved unable to penetrate the armor of these Soviet designs. A more powerful gun, the 75 mm Pak 40, was being developed, but it didn't fit in the turret of the Panzer III, Germany's primary medium tank at the time.