This is a interesting topic. My first comment will be that it may appear from what I write below that I have forgotten the OP and am talking more about Russia vs Germany then the US. I havent forgotten but am addressing this aspect because it appears to me from reading this thread that several posters have a fundamentally distorted picture of the war in Russia and this in turn distorts their understanding of America’s role. You would think from reading some comments above that the Germans almost took Moscow several times and Russia was kept afloat only on a sea of western assistance, which is very far from the truth. The reality is that the Germans bit off more then they could chew and were bled white in Russia. By the time the US/Brits/Canadians got to grips with the Wehrmacht in 1944 it was a shadow of its former self.
In my view the war in Europe was won before the United States entered it, and had been lost by the Germans no later then about October 1941. There was obviously a lot of fighting and dying that remained from that point but the crucial decisions and outcomes that decided the war in Europe had occurred by that time, specifically the failure to defeat Russia.
The war could perhaps have been won by Germany before she invaded Russia but once the invasion had occurred the decisive theatre was the east. That’s where the war was won and where it was lost. Germany did not have the manpower or the economic resources to win a sustained war against Russia and so a victory for her depended upon a knockout blow. And this, despite enormous initial successes in 1941 she just couldnt pull off. Russian resistance was too strong and German resources too limited. This initial ‘easy’ period of the war in the east cost the Germans some 900,000 casualties far more then they had suffered in the entire war up to that point.
When the Germans resumed their offensive in Russia in May 1942 they were no longer strong enough to attack along the whole front as they had in 1942 and attacked only in the southern sector. Before they did this they had to strip most of the divisions in central and northern Russia of such motor transport as they had in order to replenish the southern divisions before they were capable of offensive operations. This has been called ‘progressive de-modernisation’ and was a trend that continued throughout the war. The Wehrmacht got less and less mobile and more reliant on horse-drawn transport as the war progressed, at the same time it also possessed less and less artillery support. The overall trend was that German infantry divisions which were once a significant offensive factor in their own right became increasingly capable of only defensive operations and German offensive might was largely contained to the core of armoured and motorised divisions. The basic problems were that there were too few Germans and that German industry was incapable of making up all the losses in the east. By July 1943 they were no longer strong enough to attack even along the southern sector and just attacked a ‘bulge’ in the southern front, at Kursk and lost spectacularly. After July 1943 they had permanently lost the initiative in Russia and fought a losing defensive campaign for the rest of the war.
I’ll dig up the exact figures when I get home about the relative ‘weights’ of each theatre but a couple of figures are worth mentioning. In one calculation the amount of time German combat units spent fighting was divided into what were called ‘combat months’. The Wehrmacht spent something like 6 times as many combat months on the Russian front as compared to the Western. The overwhelming majority of all German casualties were on the Russian front and even at the peak of fighting in the west in 1944-45 about 60% of German forces were still in the east. Point being this was the most important front, most Germans that fought, fought there, and most germans that died, died there, killed by Russians not Americans or anyone else.
Lend lease isnt that significant despite RickJays claim to the contrary. Apart from the crude quantities another crucial factor is timing. By far the biggest proportion of it arrived in Russia in the post-Kursk period by which time the Soviet Union was not only well past its danger period but had already broken the back of the Wehrmacht. Compared to total Soviet production lend-lease is a small factor. The chief import of lendlease was in the provision of soft transport (400-500 thousand vehicles) which was useful in the last year of the war when most of it arrived and which allowed the Red Army to keep its offensives running for longer then they otherwise could before they would have petered out due to limited mobility. It was also useful in supplying food and aviation fuel. Deprived of this assistance the Soviet Union would doubtless have been forced to divert more resources to producing these items themselves other then armaments in which regard its important to note that the Soviet Union was at this time an industrial giant that by itself was massively outproducing Germany. The notion that the Soviet Union was only kept afloat by western aid is not to put too fine a point on it gibberish.
David M Glantz who in my view is the foremost authority now writing on the war in the east wrote in ‘Clash of Titans’ (a worthy book) that in his opinion that Russia by itself would have taken perhaps another year to defeat Nazi Germany, but the outcome was plain. He also illustrates well the relative importance of the east vs west. In July 1943 for instance the British and Americans in their sole land combat vs Germany of that time chased a grand total of 60,000 Germans off Sicily. Meanwhile two and a half-million men fought at Kursk in the biggest tank battle in history.
American fighting ability. I’d say it was essentially on par with Britain though American leadership on my readings appears to have often been more aggressive then the British who had memories of WW1 haunting their consciousness and a consequent caution to prevent casualties.
In my view, its crudely accurate to say that Russia defeated Germany and America defeated Japan, although in both cases the assistance of allies brought about the outcome quicker then would have been achieved alone.
Final comment in response to December who posits his opinion that without the expectation of US economic aid he doubts Stalin would ever have split from Hitler. This is quite bizarre. The split was caused by 3 million armed men crossing his border, expectations of US anything had nothing to do with it.