In my opinion the Soviets had already sealed their own fate early on.
1939
August 23 - Nonaggression Pact between Germany and Moscow signed in Moscow. A secret protocal drawn up by Hitler and Stalin partitions Poland between them.
September 17- USSR invades Poland with a million troops to "protect it's Byelorussian and Ukrainian population." The Poles, fighting the Nazis, are not expecting a "stab in the back." Blitzkrieg The Red Army suffers 3,000 killed, but captures 240,000 Polish soldiers and 15,400 officers.
The Polish Air Force scores its last kills: a Dornier bomber and a Soviet fighter.
September 17 - 25 - Thousands of Poles are slaughtered in eastern Poland by elements of Ukrainians, Byelorussians and Jews, encouraged by Soviet slogans such as: "For Poles, Pans and dogs--a dog's death!" In Karczowka, 24 Polish settlers are tied with barbed wire and are shot or drowned. In Luboml County, 500 Poles are murdered in a 3-day orgy by Ukrainians. A wounded priest in Brzezany County is placed near a road and is used by passing Soviet soldiers as target practice; a dozen bullets hit him before he dies at the end of the day.
September-November - The NKVD ships 15,400 Polish officers to prisons at Kozielsk, Ostashkov and Starobielsk in the USSR.
September 18 - In Grodno, a number of local Jews (some operating Soviet tanks) and Byelorussians attack the Polish administration and murder Poles. About 300,000 Jews (mostly political activists) flee the Nazis to eastern Poland where they greet the Soviets and vice-versa. The Wehrmacht and Red Army stage a joint parade in Brest Litovsk.
September 19 - Lavrenti Beria sets up a Directorate for Prisoners of War and establishes camps for 240,000 Polish POWs in Soviet custody; about 37,000 will be used as forced-labour.
September 20 - 21 - The Polish defenders of the city of Grodno kill some 800 Red Army soldiers and destroy ten tanks.
September 22 - A Polish regiment repels attacks by forty Soviet tanks and infantry units at the Battle of Kodziowce. Soviet losses amount to hundreds killed and twenty tanks destroyed.
Not to mention the Soviets were usually busy killing their own as history teaches us. < /rant >