I present to you my new TL. I hope everyone enjoys it

.
A Shift in Alliances
Chapter I: Stalin Turns East, May-October 1939.
Soviet-Japanese tensions dated back to 1931, when Japan had occupied Manchuria and began turning its interest to bordering Soviet areas, resulting in several border clashes. Japan and its puppet Manchukuo maintained that the border between Manchukuo and Mongolia was the river Khalkhin Gol which flows into Lake Buir. The Soviet Union and the Mongolian People’s Republic believed the border to run some 16 kilometres east of the river, just east of Nomonhan village.
The principal occupying army of Manchukuo was the Kwantung Army of Japan, consisting of some of the best Japanese units in 1939. However, the western region of Manchukuo was garrisoned by the relatively newly formed 23rd Infantry Division at Hailar under General Komatsubara and included several Manchukuoan army and border guard units all under the direct command of Sixth Army. The Soviet forces consisted of the 57th Special Corps, deployed from the Trans-Baikal Military District. They were responsible for defending the border between Siberia and Manchuria. The Mongolian troops mainly consisted of cavalry brigades and light artillery units, and proved to be effective and agile, but lacked armour and manpower in sufficient numbers. In 1939, the Japanese Cabinet sent instructions to the Kwantung Army to strengthen and fortify Manchukuo’s borders with Mongolia and the Soviet Union. Additionally, the Kwantung Army, which had long been stationed in Manchuria far from the Japanese Home Islands, had become largely autonomous and tended to act without approval from, or even against the direction of, the Japanese government.
The incident that would eventually escalate into a full blown war began on May 11th 1939 when a 70-90 strong Mongolian cavalry force entered the disputed area looking for grazing for their horses. They were driven off by Manchukuoan cavalry, who returned two days later with greater numbers and this time the Manchukuoans were unable to dislodge them. Lieutenant Colonel Azuma led the reconnaissance regiment and the 64th Infantry Regiment of the 23rd Infantry Division against the Mongolians who had to ask for Soviet assistance. In the resulting confrontation Azuma’s force was destroyed on May 28th and after that, in June, both sides began building up their forces in the area. The Japanese had some 30.000 men. The Soviets dispatched a new corps commander named Georgy Zhukov who arrived on June 5th with more motorized and armoured forces (I Army Group). On June 27th the 2nd Air Brigade successfully attacked the Soviet air base at Tamsak-Bulak in Mongolia, an attack that had not been authorized by Tokyo. Tokyo tried to rein in the Kwantung Army, but in the end ordered them to expel the “Soviet invaders.”
In July 1939, the Japanese assault began. The Japanese plan was for a two-pronged assault. The first attack would be made by three regiments plus part of a fourth: 71st and the 72nd Infantry Regiment (23rd Division), a battalion of the 64th Infantry Regiment and the 26th Infantry Regiment under Colonel Shinichiro Sumi (7th Infantry Division). This force would advance across the Khalkin Gol, destroy Soviet forces on Baintsagan Hill on the west bank, then make a left turn and advance south to the Kawatama Bridge. The second prong of the attack would be the task of the 1st Tank Corps (Yasuoka Detachment), consisting of the 3rd and 4th Tank Regiments, plus a part of the 64th Infantry Regiment, a battalion from the 28th Infantry Regiment, detached from the 7th Infantry, 24th Engineer Regiment, and a battalion from the 13th Field Artillery Regiment, all under the overall command of Lieutenant General Yasuoka Masaomi. This force would attack Soviet troops on the east bank of the Khalkhyn Gol and north of the Holsten River. The two Japanese thrusts were to join together on the wings. Zhukov anticipated Japanese moves and held them back, thoroughly trashing the Yasuoka Detachment, leading to the dismissal of Yasuoka. Repeated Japanese attempts failed to break Soviet lines, though they inflicted high casualties (which were easily replaced). The two sides continued to spar along a four kilometre front.
In August, Zhukov counterattacked. Utilizing a fleet of 4.000 trucks assembled from across the Trans-Baikal Military District, he transported supplies to the front from the nearest base, Chita (some 600 km away). His forces consisted of three rifle divisions, two tank divisions and two more tank brigades, two motorized infantry divisions and 550 fighter planes and bombers (the Mongolians committed two cavalry divisions). General Komatsubara’s 23rd Infantry Division with attached forces was equivalent to two light infantry division and its headquarters was at Hailar, 150 kilometres away from the fighting. Japanese intelligence, despite demonstrating the capability to accurately track the build-up of Zhukov’s force, failed to precipitate an appropriate response from below. Thus, when the Soviets finally did launch their offensive, Komatsubara was caught off guard. Initial Soviet probing attacks went badly, but they ensured that Japanese casualties continued to mount and made the disorganized state of the Sixth Army an issue of major concern. That and the fact that Japan stated it didn’t wish to escalate the incident allowed the Red Army to hand-pick elite units without having to worry about Japanese retaliation elsewhere. Soviet centre units pinned down Japanese units while Soviet armoured units swept the flanks and achieved a double envelopment.
By August 31st, Japanese forces on the Mongolian side of the border were destroyed, leaving remnants of the 23rd Division on the Manchurian side. The Soviets had achieved their objective. Lieutenant General Michitaro Komatsubara refused to accept the outcome and prepared a counteroffensive. Prime Minister Nobuyuki Abe planned to put a stop to this and de-escalate the border conflict, but Komatsubara appealed to the (admittedly declining) Kodoha political faction in the army, which was known to support the “Strike North” strategy against the USSR. Noboyuki was killed under mysterious circumstances that were never enlightened. He was replaced as Prime Minister by General Kenkichi Ueda, former commander of the Kwantung Army. Ueda was a strong believer of the “Strike North” or “hokushin-ron” policy that maintained that communism was Japan’s main enemy and that the nation’s destiny lay in conquest of the natural resources of the sparsely populated north Asian mainland. He opted to send reinforcements to the Mongolian border for a counteroffensive.
While Zhukov defeated the Japanese forces on Soviet territory, Joseph Stalin had made a deal with Nazi Germany. After the Soviet success at Nomonhan, Stalin decided to proceed with the Molotov-Ribbentrop Pact, which was announced on August 24th (and which contained a secret protocol concerning spheres of influence in Eastern Europe and, in particular, detailed how the Soviets would invade Poland together with Nazi Germany). The regime change in Tokyo was very inconvenient for Stalin because he had to concentrate his army on the invasion of eastern Poland (and the planned invasion of the Baltic States). Stalin decided to ignore the Japanese build-up for now and press on his invasion Poland in collusion with the Nazis: on September 17th 1939 the Red Army attacked and by October 6th it was over. The annexed territories were included in the Byelorussian and Ukrainian Soviet Socialist Republics. A Soviet campaign of ethnic cleansing began with a wave of arrests and summary executions of officers, policemen and priests. The Soviet NKVD sent hundreds of thousands of people from eastern Poland to Siberia and other remote parts of the Soviet Union in major waves of deportation starting in September 1939. These were set into motion immediately to support the struggle against Japan. Shortly thereafter, Stalin compelled the governments of the Baltic States to sign mutual assistance pacts which allowed the Soviets to establish military bases. Estonia, Latvia and Lithuania succumbed to Soviet pressure in June 1940, despite the Soviets being heavily committed in Manchuria at the time.
When, after September 1939, it didn’t seem like the Japanese were going to let bygones be bygones and instead escalated further, Stalin saw himself forced to do something about it. Besides that, his pathological paranoia flared up again and he became convinced that Nazi Germany, Imperial Japan, Italy and their puppets would jointly attack the USSR. After all, Germany, Japan and Italy formed the Axis powers, based on the Anti-Comintern Pact, which was explicitly anti-communist. Moreover, Hitler had explicitly written how he wished to destroy communism and conquer Lebensraum at Russia’s expense. War with Germany was therefore inevitable at some point, while a war with Japan was de facto ongoing.
Over the autumn of 1939, the Soviet dictator came to view the Anti-Comintern Pact as an aggressive alliance not just aimed at containing but also at, eventually, destroying the USSR in a two-front war despite Berlin’s current friendly tone. Officially, the Anti-Comintern Pact would only be activated in the event of Soviet aggression, but Stalin convinced himself the pact and current Japanese actions were all part of Hitler’s greater geopolitical plan, as if Japan was in on some kind of conspiracy. By October 1939, Stalin had convinced himself that he had to break the encirclement of the Soviet Union by defeating Japan and driving them out of Manchuria and into the sea before the Nazis could defeat Great Britain and France. In the meantime, to sow discord in the Anti-Comintern Pact, Moscow would adhere to the Molotov-Ribbentrop Pact by continuing to send raw materials deliveries to Germany (for now). There was nothing else to it than a preventive war so that when the fascist invasion from the West came, the fascists from the east couldn’t stab the “proletarian workers’ paradise” in the back.