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. It was found, however, that the turretless StuGs had enough room in the crew compartment to handle the 75 mm Pak 40, and this modification was made. The new vehicle proved to be an effective tank destroyer. Not only was its main gun powerful enough to knock out the new Soviet tanks, but the Panzer III chassis on which it was based was highly mobile and reliable, and the increased armor plating combined with its low silhouette made it a difficult vehicle to destroy. The StuG III became Nazi Germany's most produced armored fighting vehicle during World War II, with some 10,000 examples manufactured.