Mongolia in World War II

Outer Mongolia — officially the Mongolian People's Republic — was ruled (1930s to 1952) by the communist government of Khorloogiin Choibalsan during the period of World War II and had close links with the Soviet Union. Most countries regarded Mongolia, with its fewer than a million inhabitants, as a breakaway province of the Republic of China. Throughout the 1941–1945 war between Germany and the Soviet Union, Mongolia provided the Soviets with economic support—such as livestock, raw materials, money, food and military clothing—violating Mongolian neutrality in favor of the Allies. Mongolia was one of two Soviet satellite states not generally recognised as sovereign states at the time, along with the Tuvan People's Republic; both of these republics participated in World War II.

Pre-war Soviet–Mongolian alliance against Japan

Soviet–Mongolian relations were governed by a "gentlemen's agreement" from 27 November 1934, which was formalised in a mutual assistance pact on 12 March 1936. This treaty created a mutual defensive military alliance, and also pledged both parties to remove troops from the territory of the other when the need for military assistance had passed. These agreements were directed at Japan, which had occupied Manchuria and advanced into Inner Mongolia, and had as their object the protection of the Soviet Trans-Siberian Railway.

On 13 August 1937, as part of their effort to support China in its war with Japan, the Soviets decided to station troops along Mongolia's southern and southeastern frontiers. To obtain the Mongolian government's consent, elaborate Japanese invasion plans were forged. On 24 August the Soviet deputy minister of defence, Pyotr Smirnov, and a small staff arrived in Mongolia to oversee the transfer of the Soviet 17th Army. The arrival of the Soviet army coincided, as planned, with a series of intensified terrors and purges (the "Great Terror").