Second Communist country in the world, known as the People's Republic of Mongolia. Allinged with Russia, Mongolia had a role at the start and end of the Second World War.

A border dispute with the Japan occured in May 1939 in the far eastern portion of Mongola at Nomohan, east of the Khalkhin Gol river. An undeclared war occured between the Japanese Army versus Mongolian and Russian forces. Known to the Soviets as the "Battle of Khalkhin Gol", and to the Japanese as the "Namonhan Incident".

At the end of the WWII, Soviet-Mongolian joint army entered Southern Mongolia in order to expel the Japanese army. After the war, Stalin handed Southern Mongolia to China according to the Yalta treaty.