If they do so, they win the war in either 1940 or 1942/1943, depending on if France still falls or not. Even the worst case scenario produces far fewer Soviet casualties.
If Stalin does decide to attack, or decides to at least keep such an option open, he will order whatever preparations can be done to the transport infrastructure in Poland sped up. Such an attack would likely be launched once the Germans begin their attack west.
And yes, it's possible the Germans might detect the preparations. So what? Are they going to sit idly by in the west as a result, and await their inevitable starvation and defeat? Or are they going to weaken an operation that already had razor-thin margins anyway?
Best-case scenario for the Germans:
- Attack in the west goes as OTL
- Trade with the USSR is cut off.
- Stalin invades, reaches the Vistula (and crosses it in several locations) and the outskirts of Kongisberg.
- Forces begin trickling in from the west, halt Soviet advance.
- France surrenders as OTL instead of trying to fight on
- More forces arrive, Vistula bridgeheads are eliminated.
- Most of the Heer and Luftwaffe relocated from the west, supplies of fuel and ammo built up for an attack - the narrowness of the front (compared to OTL) and the resulting high concentration of troops means you can't just sent one or two Panzer divisions willy-nilly wherever you see an opening - it has to be a concerted effort
- Counter-offensive is launched, cuts off upwards of 1 million Soviet soldiers, pre-invasion border reached.
- Soviet pocket is eliminated
- Soviets bring in second echelon formations
Ohh, would you look at that - it's October and the rains start falling. Better wait for them to stop, no?
- It's stopped raining, and the Germans launch another offensive, trap another million men, reach the Stalin line
- Soviets bring in third echelon
Dammit, winter's here already? Ok, fine, bring in winter clothes.
Now, let's look at the strategic situation a bit:
1. Britain has likely not withdrawn from Narvik. With Soviet control of the Baltics, that means the only iron ore Germany has gotten in the last 6 months have been plundered French stockpiles
2. With Germany in a life-or-death struggle, Romania won't be bullied so easily, meaning a pro-axis government doesn't come to power, meaning they stay neutral
3. Instead of the months-long rest the Luftwaffe enjoyed OTL prior to the BoB, here they have been thrust into more high-tempo operations literally seconds after France fell. Think losing 36% (more if the French get this silly idea to drag things out) of your combat strength over France is bad enough? Try continuing that for another 4 months, fighting off thousands of (admittedly crappy) Soviet aircraft not caught on the ground... By the time the rains set in, the LW will be a shell of its former strength.
However, all of the above is pocket change when one considers the following:
4. Supplies.
Germany received from the Soviet Union in the period prior to Barbarossa:
- 1,600,000 tons of grains
- 900,000 tons of oil
- 200,000 tons of cotton
- 140,000 tons of manganese
- 200,000 tons of phosphates
- 20,000 tons of chrome ore
- 18,000 tons of rubber
- 100,000 tons of soybeans
- 500,000 tons of iron ores
- 300,000 tons of scrap metal and pig iron
- 2,000 kilograms of platinum
Large amounts of crude oil were delivered, with German documents in 1940 already indicating that the Soviets had delivered crude oil at a rate of 150,000 tons a month for five months in 900 German tank cars exclusively reserved for it.
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Moreover, they would have run out of almost everything of importance:
You will also notice that between June '41 and October '41 (5 months), Germany consumed as much oil as they had stockpiled prior to the Soviet trade kicking in. ITTL, Germany runs out of petrol sometime in mid '41, considering it doesn't get the year-long pause it had OTL and Romania is likely still neutral.
That won't matter much though, because there won't be any rubber tires for all the trucks in the first place. That, and a massive Europe-wide famine, as the German army hasn't occupied Ukrainian grain fields to live off from.
Honestly, I'd be very surprised if Hitler doesn't suffer some unfortunate accident at this point.