Most historical scholars agree it was when Nazi Germany went madcap into Russia that all hope was lost for their cause. Nazi Germany probably still would have lost over the long term (the Battle of Britain was a huge sting to their pride), but they would have made Europe into a bloody graveyard. They basically had 2 million men taken off the battlefield for good in Russia, an enormous percentage of Germany's men. Consider how many of German's human resources Russia destroyed versus that of the UK or the USA. It's not even close! Russia destroyed alone literally two times the amount Germany lost in the entire European and African front combined.
It was Russia that dealt the decisive blow, Russia that sapped Germany of its strength, Russia that crippled Germany's resolve. Yes, the way they "won" was still a Pyrrhic victory in many senses, but there's no doubt that Russia crippled the country irrevocably. There's no other real consideration here about the actual victory between Russia and Germany, because in practical terms it didn't matter Russia lost 20,000,000 men. They still won. They eradicated the worst threat they ever faced, and emerged an undeniable world superpower.
I'm also not sure how we can say Russia and Germany's deal in Poland was a major role in starting the war. The war was coming one way or another. You could say it sped the arrival of the war up, perhaps.
