In 1939, Manchuria was a puppet state of Japan, known as Manchukuo. The Japanese maintained that the border between Manchukuo and Mongolia was the Khalkhyn Gol (English "Khalkha River") which flows into Lake Buir, while the Mongolians and their Soviet allies maintained that it ran some 16 kilometres (10 miles) east of the river, just east of Nomonhan village.

From

Battle of Khalkhin Gol - Wikipedia, the free encyclopedia

Sounds like another battle of " I decide where the border goes ".

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