Japan’s invasion of Manchuria in 1931 led to border disputes with Soviet-allied Mongolia. The Mongolia–Manchuria border became a highly contentious issue, with the Japanese considering it to be demarcated by the Khalkin-Gol River, whilst the Mongolians and Soviets maintained that it ran east of the village of Nomonhan. When a Mongolian cavalry unit and their grazing horses crossed the river on 11 May, a series of skirmishes and escalations led to a build up of thousands of troops in the area. Full-scale battle broke out on 20 August when Soviet General Georgy Zhukov gave the order for an attack. A large force of Russian tanks was covertly amassed prior to the attack and used during a flanking manoeuvre which trapped Japanese forces on 25 August. The efficiency of the Soviet supply lines, which crossed some 400 miles (645 km) from the nearest railhead, was a major advantage and the Japanese were fully removed from the area by 31 August.
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