The Battle of Halhin Gol, sometimes spelled and alternately known as the Nomonhan Incident in Japan, was the decisive engagement of the undeclared Soviet-Japanese Border War (1939), or Japanese-Soviet War. It should not be confused with the conflict in 1945 when the USSR declared war in support of the other Allies of World War II and launched Operation August Storm.

Background

In 1939, Manchuria was a client state of Japan, known as Manchukuo. The Japanese maintained that the border between the two states was the Halha River (also known in Russian as the Halhin Gol, or the Khalkhin Gol), while the Mongolians and their Russian allies maintained that it ran some 16 kilometres (10 miles) east of the river, just east of Nomonhan village.