In early December of 1941, the German army began its final assault on Moscow advancing through snow, ice and bitter cold. On the night of December 5/6, the Soviet army launched a counterattack which over the next six weeks pushed the Germans back 200 miles and brought their army to the brink of collapse. Many historians consider it the military turning point of WWII.
A large part of the Soviet army's success was attributable to a battle that took place 5,000 miles away, in which the critical event happened on August 20, 1939, three days before Hitler and Stalin signed their Nonaggression Pact, and twelve days before Germany invaded Poland triggering WWII; a battle that never officially happened, a battle between two countries that were never at war - the Soviet Union and the Empire of Japan, a battle that few in the West have ever heard of - the Battle of Khalkin Gol.
After ending two centuries of isolation in the mid-19th century, Japan embarked on an expansionist agenda in East Asia, occupying Korea in 1910 and establishing a military presence guarding railroads in the Chinese province of Manchuria. In 1931 the Japanese army staged a coup and established a puppet government in the province and then, in July 1937, created an incident at the Marco Polo Bridge outside Beijing that triggered a larger war with China.
Japan and the Soviet Union (and its predecessor, the Russian Empire) had a history of hostility going back to their war of 1904-5 in Manchuria which ended with a humiliating defeat for Russia. During the Russian Civil War (1918-22), the Japanese army occupied the Russia's Far East province for several years, incurring the wrath of the new Soviet government. Although the Japanese withdrew in the early 1920s the army continued to see resource-rich Siberia as a natural area of expansion for their country.
In the later 1930s tensions grew between the two countries over the demarcation of the border between Manchuria and the Soviet Union (border disputes in the same area led to military clashes between the Soviets and the Chinese in the late 1960s and early 1970s). In addition, the Japanese were aware that in 1937-8, Stalin purged the Soviet Army removing 3 of 5 Marshals, 13 of 15 army commanders, 50 of 57 corps commanders and 154 of 186 division commanders (many of whom were executed or died in prison camps) and believing this had weakened the army were looking for a chance to test the mettle of the Soviets.
The first clash, the Battle of Khasan Lakes, took place from July 29 to August 11, 1938 and resulted in a Japanese defeat but relatively small forces were involved. Khalkin Gol was to be on a much bigger scale.
While Manchuria was a Japanese puppet state, the Mongolian People's Republic served the same role for the Soviet Union. Manchuria abutted both the Soviet Union and Mongolia and Soviet troops were stationed in both countries and borders among all of them were in dispute.
On May 11, 1939 a Mongolian cavalry unit entered the disputed area and was attacked by Japanese cavalry. Both sides began building up their forces and in June, General Georgy Zhukov was dispatched to command the Soviet effort and heavier attacks were undertaken along the Khalkin Gol river. Significant fighting continued into July and a stalemate ensued.
To break the deadlock, the Japanese planned a major assault for August 24, but Zhukov beat them to the punch by attacking on August 20. Led by a massive Soviet fighter and bomber strike and with a well designed and executed battle plan, the Soviets smashed the Japanese forces surrounding many of them at Nomonhan village (the alternative name by which the battle is known) and by August 31 had achieved complete victory. The Japanese and Soviets signed a ceasefire in Moscow on September 15. Soviet casualties were about 25,000 and Japanese losses may have been twice that amount with each army losing about 200 aircraft.(Captured Japanese soldiers)
The ramifications of Khalkin Gol affected the course of WWII. The Japanese Army continued to advocate for a strike north into Siberia. But its decisive defeat by a supposedly weakened Soviet foe helped the Imperial General Staff and the Emperor of Japan to decide instead upon a strike south in 1941 and a policy of neutrality during the German-Soviet conflict which started on June 22, 1941. The strike south towards the oil fields of Dutch Indonesia required the neutralization of the American forces in the Philippines and of the naval base at Pearl Harbor bringing the U.S. into WWII and leading to Japan's catastrophic defeat in 1945.
The Soviets continued to maintain large forces, including some of its best troops, in Siberia for two years after Khalkin Gol. Even after the Nazi surprise attack in June 1941, Stalin insisted on leaving most of the forces there, despite the pleas of his military staff, though the Russians lost 4 million soldiers during the first 5 months of the war and were near the breaking point. Then, in the late fall of 1941, Stalin learned from a Soviet spy in Tokyo that the Japanese, still intimidated by their defeat in 1939, would definitely not attack Russia and he authorized the transfer of troops from Siberia to the Moscow front. These soldiers turned the tide in December 1941 with some of them disembarking from several days on a train to rush directly to combat at the front.
One other byproduct of Khalkin Gol was the rise of Georgy Zhukov. Zhukov's performance in the battle gave him credibility with Stalin who desperately needed capable generals after eliminating so many during the purges. Zhukov went on to become the most famous Soviet general of WWII, commanding at both Moscow and Stalingrad and during the final assault on Berlin (Stalin made sure to diminish his prominence as soon as the war ended, though he proved too popular to kill).