The Battle of Halhin Gol, sometimes spelled Khalkhin Gol, was the decisive engagement of the undeclared war between the Soviet Union and Japan in 1939. The battle convinced the Japanese high command that war with the Soviet Union would be futile, and hence decisively swung opinion from the strike north faction to the strike south, who argued that Japan should capture the Pacific islands and southeast Asia. This set the stage for the attack against Pearl Harbor, a few years later. It was also the first victory won by the famed Soviet general Georgi Zhukov.

In May of 1939, Japanese forces entered territory belonging the country of Mongolia from positions in their colony of Manchukuo. This angered the Soviets, who had occupied Mongolia in the 1920s and were virtual rulers of it. The two nations had been disputing the border between Mongolia and Manchukuo for several years before the Japanese forced the issue.

The Soviet dictator Joseph Stalin ordered STAVKA, the Red Army's high command, to develop a plan for a counterstroke against the Japanese. To lead the attack, Zhukov, a young officer of promise who had escaped Stalin's disastrous purges of a few years earlier, was chosen.

The Japanese and Soviet forces engaged in limited contact throughout most of the summer. Finally, in mid-August, Zhukov decided it was time to break the stalemate. He deployed approximately 50,000 Russian and Mongolian troops of the 57th Special Corps to defend the east bank of the Halhin Gol River, then crossed the river on August 20 to attack the elite Japanese Kwantung Army with three infantry divisions (70,000 men in all), massed artillery, a tank brigade, and the best planes of the Red Air Force.

Zhukov's victory was quick and complete, in large part because the Japanese were unaware of the tank brigade Zhukov had, not to mention the potency of Russian tanks. The Soviets quickly surrounded two Japanese divisions and scattered the rest. On August 27, the Japanese attempted to break out of the encirclement, but failed. When the surrounded forces refused to surrender, Zhukov wiped them out with artillery and air attacks. The battle ended August 31 with the complete destruction of the Japanese forces.

Following the battle, the Red Army attacked what remained of the Japanese forces and drove them back into Manchukuo. On September 15, the Japanese asked for a cease-fire and later signed a treaty in which they agreed to abide by the existing border.

Soviet casualties were approximately 7,000 men killed and 11,000 wounded. The Japanese lost 61,000 men, almost all of them killed.