Without American grain shipments, the loss of Ukraine (who wanted to be freed from the soviets, those people were happy they came), the Soviet breadbasket, would have severely weakened the USSR's war capabilities. Likewise, the Western Allies primarily provided the Red Army's motor transport and lots of other things. These supports allowed the Soviet Union to deploy more military-age men in combat roles instead of agricultural or industrial work. Imagine if the West were not willing to do that. This or other differences could have changed the outcome of the war. Things happened as they did, but the USSR was not unbreakable.

It was even more crazy in the Pacific, where single engagements could have turned the tides. Where it even comes down to single spotting of fleets and assessing which fleet that is based on pure luck essentially. I would say Midway was the last chance they had.