The same could be argued for Stalingrad had the Germans prevailed, they would have had a lodging there for a while but the Soviet war machine was just getting started. The danger was if the Germans had managed to secure the Caucusus on its oil supplies. Stalin MIGHT have made a deal deal.
I think with this one it is hard to say whether the Battle of Moscow or the Siege of Leningrad or the Battle of Stalingrad was THE pivotal battle. The Soviets *MIGHT* have persisted without Leningrad and/or Moscow and Stalingrad *MIGHT* be too late because by then, as you pointed out, the Soviet war machine was getting up to speed.
I guess I'd say that somewhere between Barbarossa and Paulus' surrender was pivotal. I think it is possible that the Soviets could have collapsed just as Russia collapsed in WWI. A big part of the reason that they didn't was that the German army had to keep switching targets/focus as Hitler kept changing his mind. A secondary reason is that the Germans invaded on a HUMONGOUS front stretching from the Baltic Sea in the North to the Black Sea in the South and ultimately expanding from Lake Ladoga in the North nearly to the Caspian Sea in the South.
If they had focused more on one objective (Leningrad in the North, Moscow in the center, or the Caucuses in the South) they would undoubtedly have been able to achieve that ONE objective in 1941 rather than coming up just short on all three. Considering the three:
Leningrad:
Taking this City probably wouldn't have mattered much. In the event they had it surrounded for more than two years, destroyed most of it and caused literally hundreds of thousands of casualties and the Soviets were able to continue the war. The major impact of taking Leningrad would have been that the German troops surrounding the City would have been freed up for use elsewhere such as . . .
Moscow:
Some people will tell you that losing Moscow wouldn't have been a big deal for the Soviets because they could have just kept retreating and there is famously a LOT of land East of the Ural Mountains. In some respects that is true but it ignores the fact that as a centrally planned economy everything in the Soviet Union was centered on Moscow. It was not only their largest City, and Capitol but it was also:
- The hub of their transportation network, all rails led to Moscow.
- The hub of their communications network.
- Their largest and most productive industrial City.
- The equivalent of their banking/financial center.
Losing Moscow for the Soviets would have been something akin to the US losing Washington, NYC, Detroit, and Chicago all at once. Additionally there is the psychological impact. Stalin was hardly a beloved leader in large portions of the Soviet Union and if he had lost his Capitol and communications/transportation hub it isn't all that unlikely that the USSR would have disintegrated into revolution just as Russia did (without losing Moscow) about 30 years earlier.
Caucuses:
My view is that this would have been the best strategic target for the Germans in 1941. As I stated above, taking Leningrad wouldn't have been THAT much different that surrounding and destroying it. Moscow is a big gamble. If taking Moscow results in the USSR collapsing then that is great for the Germans but if it doesn't then they still have to keep their army fueled and there isn't any oil in Moscow. There is Oil in the Caucuses, a lot of it. Getting that oil does two things:
- It provides MUCH-needed Oil for the Wehrmacht, and
- It deprives the Soviet Military of the same.
#2 is obviously a lot easier than #1. In order to accomplish #1 the Germans would have needed all of the following:
- Possession of the oil fields in good working order, and
- Possession of a secure method to transport the crude from the fields to refineries somewhere, and
- Possession of refineries, and
- Possession of a secure method to transport the finished petroleum products to where they are needed.
In order to accomplish goal #2 all that is needed to deprive the Soviets of any one of the four steps to accomplishing goal #1.
The Soviets did have some minor other oil supplies and certainly the US would have ramped up Lend-Lease Oil but even the US didn't have unlimited transport capacity. Any ship carrying petroleum products wouldn't have been able to carry whatever else the Soviets needed.
Getting the Caucuses is much less of a gamble than getting Moscow because even if the Soviets keep fighting, their ability to fight is significantly diminished because T34's don't run on water and meanwhile the German ability to fight is significantly improved because BF109's are much more effective when they have gas.