Soviet – Japanese border conflicts The Soviet – Japanese Border Wars were a series of border conflicts between the Soviet Union and Japan between 1932 and 1945 . Before Japanese occupation of Manchukuo , the Soviet Union had conflict with China on the border of Manchuria ( See Sino – Soviet conflict ( 1929 ) ) . After the occupation of Manchukuo and Korea , Japan turned its military interests to Soviet territories and conflicts occurred frequently on the Manchurian border . Small battles . Imperial Japanese Army recorded 152 minor incidents on the border of Manchuria between 1932 and 1934 . The number of incidents increased to over 150 per year in 1935 and 1936 , and the scale of incidents became larger . In January 1935 , the first armed battle , occurred on border between Mongolia and Manchukuo . Scores of Mongolia cavalry engaged with Manchuko army patrol unit near the Buddhist temple Halhamiao . Manchuko Army incurred slight casualties , including a Japanese military advisor . Between December 1935 and March 1936 , the ( ) and the ( ) occurred . In these battles , both Japanese Army and Mongolian Army used a small number of armored fighting vehicle s and military aircraft . In June 1937 , ( ) occurred on the Amur River ( Soviet – Manchuko border ) . Three Soviet gunboat s crossed the center line of the river and occupied Kanchazu island in the river . The IJA 1st Division sunk one of the Soviet gunboats by artillery fire and damaged another . The Japanese Ministry of Foreign Affairs protested and Soviet soldiers abandoned the island . Battle of Lake Khasan . The Battle of Lake Khasan ( July 29 , 1938 – August 11 , 1938 ) and also known as the Changkufeng Incident ( , Japanese pronunciation : Chōkohō Jiken ) in China and Japan , was an attempted military incursion of Manchukuo ( Japanese ) into the territory claimed by the Soviet Union . This incursion was founded in the beliefs of the Japanese side that the Soviet Union misinterpreted the demarcation of the boundary based on the Convention of Peking treaty between Imperial Russia and the former Qing - Dynasty China ( and subsequent supplementary agreements on demarcation ) , and furthermore , that the demarcation markers were tampered with . Battle of Khalkhin Gol . The Battle of Khalkhin Gol , sometimes spelled Halhin Gol or Khalkin Gol after the Halha River passing through the battlefield and known in Japan as the Nomonhan Incident ( after a nearby village on the border between Mongolia and Manchuria ) , was the decisive engagement of the undeclared Soviet – Japanese Border War ( 1939 ) , or Japanese – Soviet War . It should not be confused with the conflict in 1945 when the USSR declared war in support of the other Allies of World War II and launched Operation August Storm . Soviet – Japanese Neutrality Pact . As a result of the Japanese defeat at Khalkhin Gol , Japan and the Soviet Union signed on April 13 , 1941 , a Neutrality Pact , similar to the German – Soviet non - aggression pact Later in 1941 , Japan would consider breaking the pact when the German Third Reich invaded the Soviet Union ( Operation Barbarossa ) at the start of the Great Patriotic War , but they made the crucial decision to keep it and to continue to press into Southeast Asia . This was said to be largely due to the Battle of Khalkhin Gol . The defeat there caused Japan not to join forces with Germany against the Soviet Union , even though Japan and Germany were part of the Tripartite Pact . On April 5 , 1945 , the Soviet Union unilaterally denounced the neutrality pact , noting that it would not renew the treaty when it expired on April 13 , 1946 . Four months later , prior to the expiration of the neutrality pact , and between the atomic bomb ings of Hiroshima and Nagasaki , the Soviet Union declared war on Japan , completely surprising the Japanese . The Soviet invasion of Manchuria was launched one hour after the declaration of war .