PzKpfw III
During Operation Barbarossa, the German invasion of the Soviet Union in 1941, it was discovered that the Soviet T-34 tank outclassed the Panzer III and IV. Its sloped armor could withstand most German weapons, and its 76.2 mm gun could penetrate the armor of all German tanks. This forced the Germans to improve their existing models. The Panzer III, which was intended to be the main medium tank, was upgraded to a longer, higher-velocity 50 mm gun.