Most Americans know very little about the eastern front during World War 2. They know even less about Khalkhin-Gol and the conflict between Russia and Japan:
In August 1939, just weeks before Hitler invaded Poland, the Soviet Union and Japan fought a massive tank battle on the Mongolian border — the largest the world had ever seen.
Under the then unknown Georgy Zhukov, the Soviets won a crushing victory at the batte of Khalkhin-Gol (known in Japan as the Nomonhan Incident). Defeat persuaded the Japanese to expand into the Pacific, where they saw the United States as a weaker opponent than the Soviet Union. If the Japanese had not lost at Khalkhin Gol, they may never have attacked Pearl Harbor.
The Japanese decision to expand southwards also meant that the Soviet Eastern flank was secured for the duration of the war. Instead of having to fight on two fronts, the Soviets could mass their troops — under the newly promoted General Zhukov — against the threat of Nazi Germany in the West.