Andy over at Siberian Lights has a nice little history of the Battle of Khalkhin-Gol. Khalkhin-Gol was the outcome of several years of competition between the Soviet Union and Japan over the border between their respective client states, Mongolia and Manchukuo. Long story short, the Japanese pushed and the Soviets gave them a nasty bloody nose, with the consequence that conquest of Siberia looked far less appealing to the Japanese than a move south.
Andy has a good summary, so I’ll confine myself to a couple of points about the battle that I became aware of during the my dissertation research. By the time that the Japanese started pushing in earnest, Stalin was right in the middle of his bloody purge of the Red Army. The purge centered around Field Marshal Mikhail Tukhachevsky, the Soviet commander who is as responsible as anyone for the (misunderstood) operational doctrine known as Blitzkrieg. Tukhachevsky was central to the collaboration between the Reichswehr and the Red Army from 1927-1933, during which the basic tenets of modern deep battle doctrine were worked out. By 1937, Tukhachevsky had become a threat to Stalin. The Field Marshal, his immediate circle, and an ever-widening wave of Red Army officers were executed for treason, with the proximate charge usually being collaboration with the Germans. By 1938, Georgy Zhukov was one of the last of Tukhachevsky’s circle to remain alive. I read in a biography that Zhukov fully believed that he was going to his death when he was summoned by the High Command in 1938; instead, he was dispatched to Siberia to handle the Japanese. It’s certainly possible that if the Japanese hadn’t been pushing, Zhukov would have joined the rest of the braintrust of the Red Army on the wrong end of a firing squad. Zhukov ended up crushing the Japanese, and later became a participant of some note in the Great Patriotic War.
Zhukov was able to crush the Japanese in part because the purge had fallen lightest on the Red Army in Siberia. A lower percentage of officers were shot there than anywhere else in the USSR. Because the Red Army retained much of its expertise in Siberia, and because Zhukov brought many of the best surviving staff officers with him, the Russians badly outmatched the Japanese in tactical and operational effectiveness. Unfortunately for the Soviet Union, this nucleus was not sufficient to restore the full combat effectiveness of the Red Army by June 1941, although a related group of Siberian returnees (officers who had been dispatched to the Gulag rather than executed during the purge) helped transform the Red Army into the most effective military organization in the world by 1944.
In August 1945, fresh from victory over the Germans, the Red Army once again fought the Japanese. With the benefit of experience and of a massive imbalance in the quality of equipment (although it should be noted that the Red Army was pretty well equipped in 1939), the Red Army destroyed the Japanese position in Manchukuo in a matter of days.