How so? Maybe if the USA was healthier and less depressed you'd have joined the war from the get-go rather than staying in bed for the first four years. But even if you'd stayed out entirely it wouldn't have delayed the soviet defeat of Germany overmuch. If you were talking about France's failure to defend Indochina you might have more of a point, but the US doesn't exactly have a great record there either.
While it is true that Soviets did a major part of the actual combat work, they probably could not have done it without economic help from the US. If US were unable or unwilling to provide such help, it is not at all given that Soviet economy alone would have survived the onslaught. While the heroism of the Red Army people is unquestionable, one needs a lot of material resources to wage war, and without them even most heroic people can not win. Even Joseph Stalin - a man not known for being overly modest - acknowledged that.