Your characterization of the PZ III and short barreled MK IV is inaccurate. They were easily "stoppable". Just read about what happened to everybody's favorite overrated Nazi Rommel at Arras in 1940 when he ran into well trained British armored forces. They shot the heck out of the 7th Panzer Division and were only stopped by the quick placement of a few 88mm anti-aircraft guns. The PZ III and PZ IV were very lightly armored and easily penetrated by the British 2 pounder, the French 47mm and 75mm. Even the US 37mm penetrated both models at 500 yards in side, rear, and sometimes frontal engagements depending on where they were hit. The Mk III did not have much more armor than a US M5 Stuart. The key to German armored success in the early part of the war was not the vehicles but the tactics, crew training, and the fact that every German tank had a radio in it. They were not unstoppable and in fact were rebuffed on the battlefield quite often. The Matilda, Char B, and Russian T-70 were more than a match for these vehicles if crewed properly. That is why the T-34 and early Shermans were such a shock to MK III and IV crews in Russia and North Africa. By the way, in the initial year of the Barbarossa campaign 1/2 of all German tanks were Panzer IIs. The Germans were successful not because of their vehicles but because of the men and equipment in them.