David Glantz, who is probably the foremost American expert on the subject, put it something like this:Laertes wrote:Guderian wrote in his postwar book Panzer Leader that it was pretty clear from 1941 onwards that the invasion of the Soviet Union was doomed and that Germany would not win the war. The thing he rated as most important was the Soviet ability to produce immense numbers of tanks very cheaply and pour them into mobile battles on the steppe where the German lines were stretched too thin to respond effectively.
Guderian was the father of tank warfare, so I sort of trust his judgement on the matter.
The loss at Moscow meant that Hitler would not win, at least according to his terms. The loss at Stalingrad meant that he would lose, and it only remained to see what the terms were. The loss at Kursk meant that the defeat would be total.
As for lend-lease, I would say that it mostly did three things:
1) Logistical support (shoes, rations, ammunition, radios, locomotives, etc.) that meant that more Soviet factories could be set to making weapons.
2) A useful but not critical supplement in tanks, aircraft, and other weapons. It should be noted that the Shermans were consistently assigned to Guards units, indicating that they were preferred over the T-34.
3) Most importantly, the jeeps and trucks in 1944-45 meant that the great Soviet offensives that crushed Germany would be motorized. Therefore, the offensives were faster, more decisive, and cost fewer Soviet lives.
Aside from that, I would say that the profoundly inefficient strategic bombing campaign did do one thing, it destroyed the Luftwaffe in the spring of 1944. And so when Bagration began on June 22, the Soviets had air superiority. That helped a great deal.
But none of it was critical, the vast majority of lend-lease aid arrived after the war in the east had effectively already been decided. There's only one bit of US aid that could be considered decisive: the relief effort for the 1921 Volga famine, in which something like five million died (though estimates range from 1-10 million). At its height, the American Relief Administration was feeding 10-11 million a day, and it also provided the seed grain to end the famine. It doesn't seem implausible that a USSR minus several million people in 1941 falls to the Nazis, or that its failure to relieve the famine might have led to another revolution.
