The battles of the Soviet-Mongolian and Japanese-Manchu troops in the area of ​ ​ the river. Khalkhin-Gol in May - September 1939 became a prelude to World War II, and their outcome had a profound impact on subsequent events. The new book of the famous Mongolian historian, statesman and diplomat R. Bold makes it possible to look at all the circumstances of this undeclared war - at its background, course and consequences - including from the point of view of Mongolia's national interests. The author pays special attention to the consideration of the general situation in the Far East, reveals the peculiarities of relations between the USSR and the MPR. R. Bold shows the desire of part of the leadership of the MPR to strengthen the sovereignty of Mongolia, to maintain neutrality in the impending global military confrontation.