For a long time Finland had been a tough nut to crack and with the defeats from multiple fronts, this forced the Soviet Union to ignore Finland and focus more on the Ottomans, Persians, Austrians, Polish, and Ukrainians.
The tide finally changed when on August 18, the Ottomans were decisively defeated at the Battle of Gagra and the Persians were soundly trounced at the Battle of Jvari forcing both of them to abandon Georgia and regroup at Azerbaijan. Meanwhile the Ukrainian army, on their own, decided to push into Mother Russia herself and make "Greater Ukraine" happen. The Ukrainian offensive dubbed "Barbarossa" launched in September 12 and the operation went swimmingly at first and they even managed to reach all they way to Stalin's very own fortress: Stalingrad. The Battle of Stalingrad raged for up to 10 weeks and the Ukrainian army was running out of food, ammunition, and morale; while on the Soviet side this, along with the Ottoman and Persian invasions, and the Romanov marriage into the Mexican Empire proved to be a propaganda victory showing the war as a Great Patriotic War against the burgeoisie and imperalistic capitalists. With renewed morale the Soviet Red Army completely enveloped the Ukrainian army and they were forced to surrender, up to 280,000 Ukrainians were taken prisoner and this forever crippled the Ukrainian army to the point that they were now relying heavily on the Austrians, Prime Minister of Canada, Winston Churchill, remarked that the battle was "The most catastrophic blows in military history."

Battles of Gagra and Jvari

Battle of Stalingrad
For the Soviet Union this gave them the new boost of morale in fighting Poland and by November 10, the Soviets were at the gates of Warsaw.
As for Finland, Stalin was now fully confident in the Red Army and with the new strategy developed by Timoshenko, the Red Army invaded through the northern border in the Karelo Soviet Oblast, north of the Leningrad Oblast, completely bypassing the Mannerheim Line and the Finns were desperately trying to redeploy divisions to the Eastern border but they were being overwhelmed and to make matters worse the lack of soldiers at the Mannerheim Line left the border practically exposed and the Red Army used this to their benefit and attacked. By the 9 December the Soviets had captured Helsinki and declared the Kingdom of Finland abolished and the new Finnish Soviet Socialist Republic along with the Lithuanian, Estonian, and Latvian Soviet Socialist Republic.

Breakthrough of the Mannerheim Line
Stalin was now so close at defeating both Poland and Ukraine that he was now preparing to puppet them. Stalin was so confident of victory that he bragged to Georgy Zhukov that by Christmas he'll be at the gates of Vienna, Berlin, Tehran, and Istanbul; and by next year Mexico City. Oh how wrong he was.
In November 2, three brigades of the New White Army numbering 16,000 soldiers, armed with carbines and handguns meant to be concealed, arrived at Bombay there they were briefed on their mission and they were loaded up onto trains and were sent to the Kingdom of Nepal, there to Tibet and finally they arrived in the Beiyang control Wuyuan and there they received Mondragón rifles, MG08s, Saint Charmond Mondragón canons and MP18s. After that they marched to the Mongolian's People's Republic.
The Mongolian army was exclusively cavalry and with the Soviet peace with Japan, the fractured state of China, and the distant threat of the German Mexican Raj there was no need to help prop up the Mongolian army (add that with Stalinist repression) the size of the army was a laughable 20,000 soldiers.

Mongolian army
It turned out to be a gigantic mistake.
The New White Army, led by veterans of the First Russian Civil War: Anton Deniken and Grigory Semyonov, marched into Mongolia and that's when they realize there was no turning back and by December 4 the New White Army came across a Mongolian scouting party of 10 horsemen and shots were fired and 6 were killed but 4 escaped and warned the Mongolian Prime Minister and General: Khorloogiin Choibalsan, of the advancing army, so he gathered up his small army and went to confront the White Army at Khalkhin Gol.

Deniken and Semyonov

Khorloogiin Choibalsan
In December 10, the two armies encountered each other on the Khalkh River and the battle began, the Mongolian plan was to attack three pronged with infantry in the middle and cavalry on the left and right charging into the enemy.
The White Army was on the side of the river and they waited for the Mongols to cross and soon as they completed the crossing and began to charge, the White Army opened fire and a storm of shells landed on the Mongolian cavalry causing the right side to absolutely panic and buckle, while the left was surprisingly continuing the assault. As for the infantry: they were being mowed down by superior machine gun and rifle fire and heavily causalities were being taken. On the left flank disaster finally struck when a shell landed on Choibalsan's horse killing both the horse and Choibalsan. That's when morale finally collapsed and the Mongolian army retreated, well I say retreated when in reality it quickly became a disorganized rout and the retreating army tried to cross the river, while still under fire, and in the chaos many drowned.
The Battle of Khalkhin Gol was a decisive victory for the New White Army, the Mongols suffered over 16,000 casualties including the Prime Minister and anarchy in Ulaanbaatar erupted until in December 20 the New White Army entered and declared the new Russian Empire under Alexei II.
Stalin had not been briefed on the defeat at Khalkhin Gol, but when he found out of the Proclamation, all hell broke loose and Stalin went ballistic. Almost immediately a whole new set of Purges, worse then the pass ones, began called the Great Purge. Thousands of officers, soldiers, intellectuals, workers, generals, and Party members were rounded up, tortured, and executed. This caused much more damage to Stalin as hundreds were now convinced that Stalin was a paranoid psychopath and begun deflecting to the New White Army because while Nicholas II was inept and absolute monarch, he was a hella alot better than Stalin.
The Great Patriotic War had now morphed into the Second Russian Civil War.