History doesn't tolerate what-ifs, but just in case. Soviet Union wouldn't stand war with Germany without Allies. Hitler would get access to all the Soviet's resources: cheap slaves' workforce, food, oil, coal, steel, Arctic and Far East seaports. Then attack on the US together with Japan, both East and West coast. The US would not have a chance. Game over.
As far as I know, current consensus is that the Soviet Union would have defeated Nazi Germany regardless of Western intervention, it just would have taken more time (and a lot more casualties) to get to Berlin.
It really is a case where the Nazis picked a fight they just couldn't win.
Current consensus where? I live in Russia, we currently have a state-induced wave of unhealthy "patriotic" hype here, mostly based on WW2 events. But even in these circumstances I rather got an opinions like "without West we wouldn't make it against Hitler" in my circles.
The keyword is "lend-lease" [0]. About 5% of weapons for 1942 Soviet offensive operations, decisive for the final outcome, were US-made. And 80% of cars and trucks. And lots of raw material.
Consensus in the West, by historians not trying to score a political point about current world affairs.
Lend-lease, while critical help, is often overplayed in the West as a way to diminish the Soviet contribution to the war -- this is in itself the legacy of historical views shaped by the Cold War and (unsurprisingly) by ex Wehrmacht officers who collaborared with the West after WW2. There's no credible mainstream analysis that has Germany winning against the Soviet Union. In fact, it is often argued that by starting Barbarossa, Hitler effectively lost the war. Germany didn't have the manpower, logistics capabilities or equipment to take on the Soviet Union.
That's not to say it wouldn't have cost the Soviets a lot more time, effort and bloodshed to defeat the Nazi war machine without the help of the Western Allies.
I'm not Russian and I'm completely uninterested in how current Russia and Putin are spinning this, by the way.
