(Sd.Kfz.121) was larger than Panzer I but also did not prove very effective in combat, although it was the main battle tank of the Panzertruppen until 1940/41. The main armament of 20mm cannon was adequate at the time of its introduction into service but soon proved to be an outclassed weapon. After the Fall of France, due to the poor cross-country performance, some older Panzer II tanks were taken out of service, and an improved and modified version replaced them armed with newer 20mm KwK 38 L/55 cannon. But from then on the Panzerkampfwagen II tanks was phased out and the remaining chassis were used as a base for Marder II (Sd.Kfz.131) tank destroyers and Wespe (Sd.Kfz.124) self-propelled howitzers. Panzer III and Panzer IV The second generation were the more heavily armored Panzer III and Panzer IV medium tanks. Ideally, the tank battalions of a panzer division were each to have three medium companies of Panzer IIIs and one heavy company of Panzer IVs. The Germans began to convert their tank battalions to a majority of Panzer III and IV medium tanks soon after the 1940 French campaign, thereby stealing a march on the Soviets and British, who still possessed obsolete equipment. Panzer III was the first of German Panzers to be equipped with intercom system for in-tank communications. Later on all of Panzers were equipped with this device which, proved to be very effective during combat. Panzer III was designed as platoon commander's vehicle (Zugführerwagen) and was Germany's first true medium battle tank. The Panzer III formed the bulk of the Panzer Divisions’ strength during early years of war. Also, in 1940/41, attempts were made to standardize the production of Panzer III and Panzer IV but soon after further development was halted. Panzer V (Panther) However, the appearance of a few of the new generation T-34 and KV-1 tanks in Russia during 1941 compelled the Germans to begin a race for superior armor and gun power. The third generation included many different variants, but the most important designs were the Panzer V (Panther) and Panzer VI (Tiger) tanks. First encountered on 23 June 1941, the T-34 outclassed the existing Panzer III and IV. At the insistence of General Guderian, a special Panzerkommision was dispatched to the Eastern Front to assess the T-34. Among the features of the Soviet tank considered most significant were the sloping armor, which gave much