In 1938 Zhukov was directed to command the First Soviet Mongolian Army Group, and saw action against Japan's Kwantung Army on the border between Mongolia and the Japanese controlled state of Manchukuo in an undeclared war that lasted from 1938 to 1939. What began as a routine border skirmishthe Japanese testing the resolve of the Soviets to defend their territoryrapidly escalated into a full-scale war, the Japanese pushing forward with 80,000 troops, 180 tanks and 450 aircraft.
This led to the decisive Battle of Halhin Gol. Zhukov requested major reinforcements and on August 15, 1939 he ordered what seemed at first to be a conventional frontal attack. However, Zhukov had held back two tank brigades, which in a daring and successful manouvere, he then ordered to advance around both flanks of the battle. Supported by motorized artillery and infantry, the two mobile battle groups encircled the 6th Japanese army and captured their vulnerable supply areas. Within several days the Japanese troops were defeated.
For this operation Zhukov was awarded the title of Hero of the Soviet Union. Outside of the Soviet Union, however, this battle remained little-known, as by this time World War II had begun. Zhukov's pioneering use of mobile armour went unheeded by the west, and in consequence the German Blitzkrieg against France in 1940 came as a great surprise.