11 May — 31 August — The defeat of Japanese forces near the Khalkhin Gol River by the Red Army and units of the Mongolian People's Republic.
In January, the third population census (170.6 million people) was conducted and declared accurate. However, as has now been established from archival materials, the leaders of this census, fearing reprisals, deliberately inflated the results. The 1939 census had a clear goal: at any cost to show growth in the population of the USSR. Apparently, realizing how incorrect the census procedures were and how flawed the materials were, a significant part of them was moved into secret archives, and only some figures made it into the open press. According to the official version, the detailed results of this census were not published because the war had begun; however, this was not done in the postwar period either.
Each person was to be counted in person, and only in exceptional cases was information obtained from relatives or neighbors, and then compiled from house registers. Multiple visits by the enumerator were предусмотрены. Everyone who changed their place of residence during the census period was issued a certificate confirming participation in the census so that they could not evade it. For the first time in Soviet census practice, criminal penalties were introduced for evading the census. A special unofficial decree of the Council of People's Commissars of the USSR was adopted on searching for and counting, during the census, homeless and unregistered urban residents hiding in basements, asphalt-boiling kettles, attics, under bridges, and so on. As part of the general census, two special censuses were conducted: one of the special contingent (prisoners, guards at places of detention, and employees of the NKVD apparatus) by NKVD bodies, and one of military personnel by the People's Commissariat of Defense. The results of both censuses were added to the overall totals.
In this environment, it was difficult to assume any significant undercount of the population. Indeed, at first the control обходы produced no results, and in many areas no omissions were found at all. Then pressure “from above” on enumerators and inspectors was increased through “workings-over” and reprimands. Thus, a favorable situation for padding the figures was created. For example, in the documents of the Tambov census bureau, inspector Afanasyev found only 4 people missed by the enumerator during the first control round, but after a “working-over” he immediately “found” 287 uncounted people in his area.
Ahead of the upcoming census, rural населенные пункты were urgently reclassified as towns, but towns were not reclassified as settlements, although this was done before every census and was a standard procedure. In addition, decisions to reclassify rural localities as towns were allowed to be made locally, as a result of which many dacha settlements where workers lived were granted town status. As a result, a figure was obtained that showed more than a doubling of the urban population compared with 1926.
30 November — the Soviet-Finnish War began; Soviet troops entered Finnish territory without declaring war. The main fighting unfolded on the Karelian Isthmus. Finnish forces quickly withdrew to a заранее prepared defensive line from Taipale to Summa ("the Mannerheim Line").
The Soviet offensive in northern Karelia was stopped. The war dragged on. In conditions of severe морозы and strong Finnish defenses, the Soviet army suffered heavy losses. Western countries supported Finland. Sweden, England, the United States, Italy, and France supplied Finland with weapons. Germany, adhering to the terms of the Pact, not only did not assist Finland but also задержала aid arriving in transit through its territory from other countries. The League of Nations (the predecessor of today's UN) called on all member states to provide whatever assistance they could to Finland as a victim of aggression; by that time both Germany and the USSR had already been expelled from the League of Nations: the former for Poland, the latter for Finland.