This may be off topic but it’s never been difficult to see how Germany taking Moscow (and Leningrad) in 1941 could feasibly lead to the USSR losing the war. What’s murkier is how Germany taking Stalingrad (and accomplishing Case Blue’s other goals) and holding off Soviet winter counter offensives in 1942 leads to the USSR’s defeat.
What’s usually suggested is Stalin snaps and starts purging major figures in the Soviet government and military or a civil war breaks out (from Stalin’s purges, wasteful offensives, the Heer’s continued success or a combination thereof) resulting in the Red Army’s collapse and Stalin’s removal leading to someone (Molotov etc) taking control and seeking terms with the Reich (or retreating behind the Urals).
Are these the most plausible outcomes from a military/alternate history point of view or is there something I’m missing?
Stalingrad became the pivot point on the Eastern Front for a number of reasons, both material and ego related.
Material/strategic first - The Volga was one of, perhaps the most, vital waterway in the Soviet Union. Huge amounts of goods, including oil and grain traveled up and down the waterway. While not quite the level of the Mississippi trade wise, it was perhaps more important in a relatively transportation poor Soviet Union, where the road and rail network was quite weak by European (and fairly pitiful by U.S. standards, especially the road system) into the war years. Stalingrad (actually known as Volgagrad both before Stalin's rise to power and after the fall of the USSR) is ideally placed to interdict traffic on the river, especially from the oil rich Caspian Sea region. The side that holds it holds the, at the time, main source of oil for the Soviet Union (and the Red Army). Germany's greatest weakness, resource wise, was oil*, any modern industrialized country required it in vast quantities, it is literally the life blood of industry and transportation, the entire reason for the German 1942 Southern Offensive was to gain control of Baku and the Caspian Sea oil resources found nearby. Without Stalingrad movement of that vital resource back into the Reich would be vastly more difficult and even if the Caspian is not reached possession of the city (more properly the riverfront on which it is located, would be a huge step in denying the Caspian Sea oil to the Soviets).
The second strategic issue is that the battle for the city became a black hole for combat formations. Both Heer and Red Armylosses during the battle exceed the TOTAL combat losses (KIA/WIA/MIA) for the United States in all of WW II (86,000 German captured at Stalingrad died in Soviet PoW camps, total USMC deaths/died of wounds in WW II were slightly above 20,000). Neither side could make up the losses suffered there, making victory in the battle absolutely critical, since the loser would be hard pressed to have another go (this was especially true for the Germans, who were already experiencing manpower shortages to the point that they had been forced to use 100,000
lightly armed Romanians, well over 120,000 Italian, and 120,000 Hungarian troops, all with insufficient heavy equipment to cover sections of their defensive front). Failing at Stalingrad meant loss of control of the Volga, all that implied, until the end of the War.
The second factor (and in some ways, the more critical one) is the egos of the two War Lords involved. Hitler became utterly obsessed with taking "Stalin's City" for symbolic reasons and Stalin, for the same symbolism became remarkably focused on holding it. The became an increasingly critical element in the battle, by October both dictator's were laser focused on the actions, requiring daily detailed briefings on any movement of forward positions advancing or retreating (this is also the period, where any commander with an ounce of brains would have looked at the German position and withdrawn to a better defensive position to have a fresh try at the City in 1943, before throwing away a few hundred thousand additional troops). Hitler's ego and hatreds prevented him from seeing this, and he gutted the Heer as a result.
Had the Soviets lost the city the impact on morale would have been enormous; it would also very possibly have led to one of Stalin's infamous fits of anger with God knows how many purged senior officers (including the very officers who led the Red Army to Berlin 29 months later) or, alternatively Stalin committing suicide by shooting himself in the back 36 times with three different calibers of ammunition and then cutting his throat, twice. Either way the impact on the Soviet war effort is incalculable and quite possibly sufficient to turn the tide in the East.
*There were a number of others, mainly ores needed as alloying elements in high strength steel, and rubber, but even the ore situation could be, to a degree, managed by imports through third parties like Turkey, Spain and Sweden, and synthetic rubber eased, but did not erase the need for natural substance)