Mongolia in World War II

Outer Mongolia—officially the Mongolian People's Republic—was ruled by the communist government of Khorloogiin Choibalsan during World War II and was closely linked to the Soviet Union. Mongolia, with less than a million inhabitants, was considered a breakaway province of the Republic of China by most nations. Until 1945, Mongolia kept formal neutrality. Throughout the war with Germany, the country provided the Soviet Union with economic support, such as livestock, raw materials, money, food and military clothing. Mongolia was one of two Soviet satellites not generally recognised as sovereign nations at the time, the other being the Tuvan People's Republic; both participated in World War II.

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"). In his address to the Third Session of the Supreme Soviet on 31 May 1939, Foreign Commissar Vyacheslav Molotov declared that "we shall defend the frontiers of the Mongolian People's Republic just as resolutely as our own border."

Mongolia was directly involved in the battles of Khalkhin Gol that lasted from May to September 1939.

Mongolia reportedly signed an agreement with the Japanese puppet state of Manchukuo on 18 July 1940. In the Soviet–Japanese Neutrality Pact of 13 April 1941, the two powers recognised the neutrality of Mongolia and its place with the Soviet sphere of influence. Its geographical situation meant that it served as a buffer between Japanese forces and the Soviet Union. In addition to keeping around 10% of the population under arms, Mongolia provided supplies and raw materials to the Soviet military, and financed several units, for example the "Revolutionary Mongolia" Tank Brigade and "Mongolian Arat" Squadron and half million military trained horses. Also, more than 300 Mongolian volunteer military personnel fought in the Eastern front.

Mongolian troops took part in the Battle of Khalkhin Gol in the summer of 1939 and in the Soviet invasion of Manchuria in August 1945, both times as small part in Soviet-led operations against Japanese forces and their Manchu and Inner Mongolian allies. During the 1945 campaign, the Mongolian troops were attached to the Soviet–Mongolian Cavalry Mechanized Group under Colonel General I. A. Pliev. The Mongolian units were the 5th, 6th, 7th and 8th Mongolian Cavalry Divisions, the 7th Motorized Armored Brigade, the Armored Car Brigade and the 3rd Artillery Regiment. On 10 August 1945, over twenty-four hours after the first Mongolian troops in the company of their Soviet allies had crossed the border into Japanese-occupied China, the Little Khural, the Mongolian parliament, issued a formal declaration of war against Japan.

Today, the Zaisan Memorial in the southern area of the Mongolian capital of Ulaanbaatar honors the Soviet soldiers killed in World War II.