"By the summer of 1939, Japanese strength was estimated at around 80,000 soldiers, 180 tanks and 450 aircraft. Soviet strength had reached approximately 50,000 soldiers, supported by 498 tanks and armored vehicles and 581 fighters and bombers.
In July 1939, Japanese forces moved across the frontier with Mongolia and, inflicting heavy losses on Soviet and Mongolian troops, occupied the disputed border region.
On Aug. 20, 1939, upon the signing of the German-Soviet Nonaggression Pact, Zhukov launched an attack on Japanese forces in Mongolia. Using his artillery and infantry to pin Japanese forces in place, Zhukov sent his tanks to attack on both flanks of the Japanese position. The attack encircled the Japanese Sixth Army and ultimately crushed it. Roughly 75 percent of the Japanese frontline troops were killed in action. The fighting ended on Sept. 16."