During the 1930s, the Japanese military and the Red Army were fighting a undeclared war on the border between Japanese-occupied Manchuria and Mongolia, beginning when the Japanese invaded Manchuria in 1931 and intensifying when Japan invaded China in 1937.
From early 1935 until April 1939, the Japanese recorded 108 incidents where Japanese and Soviet forces engaged in small skirmishes. These included 'incidents' at Khalkhyn Temple, Inner Mongolia (part of Japanese-occupied Manchuria) in 1935, Tauran, Mongolia in March 1936 and Kanchazu Island on the Amur River in June 1937. Before Khalkin Gol, the only major battle was at Lake Khasan between July-August 1938.
The final showdown took place at Khalkin Gol. For many reasons including logistics, the fact that the Red Army was more mechanized, and the Japanese tactical doctrine including the out of date strategy of charging the enemy en-masse and the leadership of Soviet forces by future Soviet war hero Gregory Zhukov (for which he earned two Hero of the Soviet Union awards, the highest honour in the Soviet Union), the Japanese lost the Battle of Khalkin Gol.
On 13th April 1941, the Soviets and Japanese decided that enough was enough and they signed the Soviet-Japanese Neutrality Pact. The defeat at Khalkin Gol would convince Japanese military leaders that it was not a good idea to antagonize the Soviet Union, which is why they stayed out of Operation Barbarossa when Hitler invaded the Soviet Union and instead, focused on Southeast Asia.
The Soviets would later break the Neutrality Pact and invade Japanese-occupied Manchuria in August 1945 after the atomic bombing of Hiroshima.
But what if the Soviet Union somehow lost the Battle of Khalkin Gol?