From May to September 1939 Japan and the Soviet Union fought a fierce, large-scale undeclared war on the Mongolian plains that ended with a decisive Soviet victory with two important results: Japan reoriented its strategic emphasis towards the south, leading to war with the United States, Britain, and the Netherlands; and Russia freed itself from the fear of fighting on two fronts, thus vitally affecting the course of the war with Germany.

From May to September 1939 Japan and the Soviet Union fought a fierce, large-scale undeclared war on the Mongolian plains that ended with a decisive Soviet victory with two important results: Japan reoriented its strategic emphasis towards the south, leading to war with the United States, Britain, and the Netherlands; and Russia freed itself from the fear of fighting on two fronts, thus vitally affecting the course of the war with Germany.