The Soviets didn’t defeat the Japanese though, and they benefitted substantially from American supplies. As did the British, which kept the Nazis occupied in North Africa, in the air, and elsewhere.
Anglosphere people underrate the Soviet contribution but it’s not clear the Soviets could have done it alone.
Though in fairness, by November 1943 when the torpedo attack happened perhaps they could have.
The Soviets actually had an entente(?) with the japanese right up until the end of the war. Presumably, if the US hadn't asked them to break the entente, they could have just continued that indefinitely (although it's pretty likely some war would have occurred at some point). The Germany-Japan alliance was very thin (for instance, the Germans did not notify the japanese they were planning on invading russia). I think it's arguable that the eastern and western theaters of the war are essentially two separate wars that just happened at roughly (if you squint) the same time.
For what it's worth, the japanese were absolutely terrified of the USSR, for good reason - and when the russians invaded manchuria they basically threw in the towel.
That’s true, but the Japanese had transferred their best units and equipment out of Manchuria to fight in the Pacific, and the Soviets did not want a two front war.
If there had been no chance of American entry Japan probably would have attacked the eastern ussr and the soviets would have faced real difficulties.
But indeed the massively successful Soviet invasion of Manchuria was decisive in Japan’s swift unconditional surrender.
My guess is that Japan would not have done particularly well had they fought the USSR instead of the USA. The USA still blocking japanese oil imports would mean they would have horrible logistical problems. Presumably, the USA would still be supporting the Soviets through the lend-lease program, so although the USSR would be fighting on two fronts, it's totally plausible they would be winning on both fronts. At least, Stalin seemed to think so - after the battle of Stalingrad, he started making public condemnations of IJ, etc.
Even if the Japanese did do really well and pushed the soviets back, they'd just end up in the same situation they were in the 20's, where they controlled large swathes of eastern russia which were basically totally worthless, while also being extremely volatile.
If the Japanese had attacked the USSR at the same time as Germany did, and the USA had not intervened, I guess maybe the USSR would have lost? It's really hard to say: I always have the sense that the Nazis were far more fragile than they look on maps, and engulfing a huge amount of extremely rebellious territory might have just ended up like China ended up for the Japanese.
Anglosphere people underrate the Soviet contribution but it’s not clear the Soviets could have done it alone.
Though in fairness, by November 1943 when the torpedo attack happened perhaps they could have.