Would the Nazis have lost without US involvement?

The Germans had been broken at the battle of Kursk. US invasion only accelerated eventual German defeat.

Hmm, true, though if the US hadn’t entered the war, the Germans would not have had to fight the Americans in Africa or Italy, nor would they need to keep as many forces in France in case of invasion, nor would they have had to deal with US air raids, or US destroyer escorts for convoys to England, which would have had effects even before Kursk, and would have changed the level of urgency of the war with the Soviets. The Soviets also would have been impaired without Lend Lease support from the US (though it’s not clear if that was included in the question or not).

The thing with such questions, is that since circumstances depend one upon another, if much of anything changes, the later events may be very different, and can’t really be predicted.

If England was the only worry on the western front, i think it’s not out of the realm of possibility that Nazi Germany could have battled the Soviets to a standstill on the Eastern front though without a doubt after losing much of the territory they claimed from the USSR in their push eastward. But also if the US was out of the war, the USSR would have also have to deal with Japan and then the soviets could have ended up being spread too thin to battle Nazi Germany and Japan at the same time.

They already did before the US bothered to enter the war.

I think the war would have gone on for a long time but ultimately Germany would have been spread too thin to control France and the lowlands and the parts of Scandinavia they had at the peak of expansion, not to mention Poland and south central Europe.

An uncooperative population would have worn them down, the situation was not sustainable.

As others have said, it is impossible to know for sure.

To some degree it depends on the success that the Germans would have had in taking control of the conquered lands, and how coercive they would have been in enslaving the conquered countries. For example, if German had taken England (before taking the rest of France and Russia) then they would have had access to England’s significant manufacturing capacity (steel, ships, etc.). They could have taken that manufacturing ability to make more and better armaments which then they would have used for taking over Russia all the say to Asia.

But that assumes that the Germans were prepared to occupy and take lots of losses in England (which would have fought back), and it is only speculation if Germany would have thought that worthwhile.

One thing we can say for sure is that if the US had not assisted Russia (Stalin) than Russia would have been lost,

But so much of this is guesswork, because we don’t know what Germany’s strategy might have been if they had won in various battles, and lost in others.

The United States literally put the Red Army on wheels though all the lend lease trucks we shipped them. Every truck they received meant a truck they didn’t have to build at the expense of a tank. The operational/logistical depth and op-tempo of the Red Army in 1944–5 would not have been possible without them and the Germans (still reliant on horses for the bulk of their infantry divisions) would have been fighting them on more even terms.

Another factor to consider is the stress put on Germany by America’s contribution to the air war. While the attempt to completely derail German industry through destroying critical industrial plants and oil facilities did not meet with success in 1943, every fighter, flak gun and flakhelfer on defensive duty represented something or someone that couldn’t be used in the East.

And one more way to look at it: the USA rounded out the Allies. Roosevelt’s highly-liberal circle of confidants and fixers were rather more sympathetic towards the Soviet Union than were Churchill and the English establishment. It is easier to see the Allies breaking apart in the face of a German offer of a separate peace if it was just the UK/Commonweath and the USSR as the major players. Perhaps more importantly, full American commitment made the balance tipping in industrial and combat power obvious; everyone from that point knew victory was quite certain if they continued to plug ahead and this removed the temptation to seek a political out.

A few years later, yes.