You have said something important here; that before the USA was even in the war, between the Blitz and Barbarossa, that Germany punched itself out and went down the Napoleon road of bigger bets and bigger losses.
But there are some important things to consider–the material commitment being only one. You are correct and we all seem to agree that the total bulk of the Lend Lease materiel did not flow until later in the war, but no doubt the amounts sent later in the war were less critical and even more massive considering the increased capacity of the 3 routes to send it. So the physical and moral and protective support of having another productive ally was felt for several years, which prevented despair.
While Germany faced setbacks in the East, one must consider that their timetable and no-step-back approach was dictated by the necessity of ending or at least advancing the war on one front with aggressive moves before the full weight of the enemy is felt. This was the motivation for the Hundred Days Offensive in WW1, that they only had limited time before they were overwhelmed, which forced them to attempt to destroy the USSR in a reckless and incomplete fashion before they had to face the Western Allies.
Once that process began, we cannot discount the
- US contribution on the water, which prevented Axis domination of both Oceans.
- UK power was insufficient, as others pointed out an unchecked (or even separate-peace Japan) would have dominated the former UK economy and sphere
- US Strategic Bombing, while not decisive or even debilitating, required a huge and inconvenient redistribution of assets the Axis did not have (radar belts, night fighters)
4) The US ended any hope of Axis domination of the ME by giving the UK (monty) a ton of tanks and troops to fight Rommel in a war of attrition…in some situations the Allies were losing 3;1 and they still overwhelmed the Axis
5) The threat of the Western Invasion, while not critical for some years, required a substantial diversion of forces in order to dissuade a 1943 cross.
6) The failure of unrestricted submarine warfare and the wunderwaffen to produce some decisive effect were the product of limited, hopeful and defensive thinking that flowed from being attacked from all sides.
I think we can conclude that while 1941-42 the Germans would have had setbacks, huge disasters against Russia, but that the UK was effectively paralyzed and without any concern for being attacked from behind in force, that Germany could have easily recovered from these setbacks and torn Russia apart, if not taken Moscow. It is a good point that US entry into the war (in both wars) was at first partly symbolic; the psychological and strategic effect of the Axis being forced to act in time with limited resources was profound. Once Germany was being assaulted in on multiple fronts, in multiple ways, this is when its economy was forced to rationalize and this led to an extreme stretching of resources that could not be sustained (except by Russia or the US, not Germany). Think just in terms of a limited supply of oil being parsed out. The USA at that time provided a large proportion of the oil and refined products to the Allies (and the world).
The schwerpunkt/blitz method relies on maximum force applied all at once–any diversion of force at the critical time can cause the strategy to fail.