superior tactics and French blunders, not superior weapons, that made the "blitzkrieg" so successful in May 1940. For information regarding tank development in this period, see tank development between the wars. Germany, Italy and the Soviet Union all experimented heavily with tank warfare during their clandestine and "volunteer" involvement in the Spanish Civil War, which saw some of the earliest examples of successful mechanized combined arms —such as when Republican troops, equipped with Soviet-supplied tanks and supported by aircraft, eventually routed Italian troops fighting for the Nationalists in the seven-day Battle of Guadalajara in 1937. However, of the nearly 700 tanks deployed during this conflict, only about 64 tanks representing the Franco faction and 331 from the Republican side were equipped with cannon, and of those 64 nearly all were World War I vintage Renault FT tanks, while the 331 Soviet supplied machines had 45mm main guns and were of 1930s manufacture. The balance of Nationalist tanks were machine gun armed. The primary lesson learned from this war was that machine gun armed tanks had to be equipped with cannon, with the associated armour inherent to modern tanks. The five-month-long war between the Soviet Union and the Japanese 6th Army at Khalkhin Gol (Nomonhan) in 1939 brought home some lessons. In this conflict, the Soviets fielded over two thousand tanks, to the around 73 cannon armed tanks deployed by the Japanese, the major difference being that Japanese armour were equipped with diesel engines as opposed to the Russian tanks equipped with petrol engines. After General Georgy Zhukov inflicted a defeat on the Japanese 6th Army with his massed combined tank and air attack, the Soviets learned a lesson on the use of gasoline engines, and quickly incorporated those newly found experiences into their new T-34 medium tank during World War II. Prior to World War II, the tactics and strategy of deploying tank forces underwent a revolution. In August 1939, Soviet General Georgy Zhukov used the combined force of tanks and airpower at Nomonhan against the Japanese 6th Army; Heinz Guderian, a tactical theoretician who was heavily involved in the formation of the first independent German tank force, said "Where tanks are, the front is", and this concept became a reality in World War II. Guderian's armoured warfare ideas, combined with Germany's existing doctrines of Bewegungskrieg ("maneuver warfare") and infiltration tactics from World War I, became the basis of blitzkrieg