tank destroyers formed into independent tank destroyer battalions. The latter is most closely identified with the Chief of Army Ground Forces, Lesley J. McNair. Having studied the early German successes McNair came under the belief that U.S. forces would be faced with fast moving enemy forces who would seek to bypass, isolate and reduce U.S. forces in a replay of the Fall of France. To stem the flood of marauding panzers, fast moving powerfully armed tank destroyer battalions were created to be held back and used in the counter-attack. It was also calculated that U.S. interests would be better served by large numbers of reliable (battle-worthiness) medium tanks rather than a smaller number of unreliable heavy tanks. It was decided therefore to slow the production of the U.S. heavy tank designs such as the M26 Pershing and concentrate resources on mass-producing the M4 Sherman and tank destroyers such as the M18 Hellcat. To be able get into position to counter-attack, the tank destroyers had to be fast. To achieve the desired mobility and agility from the engines available the armour protection was sacrificed, a measure of protection coming from being nimble and hopefully from being able to knock out the enemy before they could get a shot in. Although they usually had guns of either 75 mm or 76 mm calibre (the M36 used a 90mm calibre gun), the tank destroyer units were issued with the ancestor of the modern armour-piercing discarding sabot, rounds which made their guns much more powerful than a simple comparison of calibres would suggest. Japan The Japanese doctrine was mainly French in concept but with some purely Japanese elements. Due to Japan's naval priorities in warship construction and inter-service feuds (the marine branch of the IJN favoured all-around protective armour) IJA tanks were lightly armoured. As with most armour during the 1930s, the main guns were small in calibre: 37 mm for their Type 95 light tanks and 47 mm for the Type 97 medium tank, but this was sometimes compensated by a high muzzle velocity. The IJA's use of tanks in China exemplifies its doctrine: light tanks were used for scouting or acted as mobile infantry support, while medium tanks supported the infantry and assaulted deeper objectives, but did not fight en masse. In 1939, the Japanese Army engaged Soviet armour at Nomonhan. During the three-month-long war, Japanese armour had shown their weakness against