appointed commander of the 4th Cavalry Division. His career was accelerated by the Great Purge, when thousands of officers were arrested and shot, but those associated with the First Cavalry Army were protected. In 1937, Zhukov became commander of first the 3rd Cavalry Corps, and later the 6th Cavalry Corps. In 1938, he became deputy cavalry commander of the Belorussian Military District. Khalkhin Gol 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 the Mongolian People's Republic and the Japanese-controlled state of Manchukuo. The Soviet–Japanese border conflicts lasted from 1938 to 1939. What began as a border skirmish rapidly escalated into a full-scale war, with the Japanese pushing forward with an estimated 80,000 troops, 180 tanks and 450 aircraft. These events led to the strategically decisive battle of Khalkhin Gol. Zhukov requested major reinforcements, and on 20 August 1939, his Soviet offensive commenced. After a massive artillery barrage, nearly 500 BT-5 and BT-7 tanks advanced, supported by over 500 fighters and bombers. This was the Soviet Air Force's first fighter-bomber operation. The offensive first appeared to be a typical conventional frontal attack. However, two tank brigades were initially held back and then ordered to advance around on both flanks, supported by motorized artillery, infantry, and other tanks. This daring and successful maneuver encircled the Japanese 6th Army and captured the enemy's vulnerable rear supply areas. By 31 August, the Japanese had been cleared from the disputed border, leaving the Soviets clearly victorious. This campaign had significance beyond the immediate tactical and local outcome. Zhukov demonstrated and tested the techniques later used against the Germans in the Eastern Front of the Second World War. His innovations included the deployment of underwater bridges, and improving the cohesion and battle-effectiveness of inexperienced units by adding a few experienced, battle-hardened troops to bolster morale and overall training. Evaluation of the problems inherent in the performance of the BT tanks led to the replacement of their fire-prone petrol (gasoline) engines with diesel engines. This battle provided valuable practical knowledge that was essential to the Soviet success in development of the T-34 medium tank used in World War II. After this campaign, veterans were transferred to untested units, to better spread the benefits of their battle experience. For his victory, Zhukov was declared a Hero of the Soviet Union. However, the