This is referring to a small scale conflict between Japan and Russia from 1938-39. It was actually a border dispute with Japan trying to expand its sphere of influence from its protectorate of Manchukuo into russian territory. Small engagements took place in 1938 with japanese troops advancing towards two russian held cities. However, after some fighting the russians were able to reinforce their troops and the conflict was ended by request of the japanese emperor.
The disputes gained new momentum in 1939, when mongolian cavalry allegedly violated the borders to Manchukuo. After small skirmishing and reinforcements of the japanese Guandong-Army the russians devised a plan to seize the disputed area permanently. Subsequently russian and mongolian forces with about 50.000 troops attacked the japanese defenders, numbering somewhat over 30.000 men. After about ten days of battle the Russians broke through the japanse defense lines. Two japanese divisions where captured, the Japanese had casualties of approx. 8500 troops, russian and mongolian losses totalled for about 6.800 and additionally 1.500 MIA.
This engagement is thought by observers to be the major reason why japanese troops did not start invading Russia after its ally Germany launched its own invasion in 1941, very much to the disappointment of Hitler.