Why the late 1920's? Because after that the Soviet Union would have gone through at least part of the Stalinist purges, and I'm trying to figure out what would have happened if those purges had gone differently. How could a civil war have happened in the Soviet Union in the late 1920's? That's actually harder than it seems at first. The Trotsky/Stalin rivalry appears to have some potential, but Trotsky was nowhere near a match for Stalin in terms of political skills, especially timing and gathering allies. Also, at this point the Communist party did have a great deal of solidarity. It had its factions but those factions tended to put aside their differences when party rule was threatened. Also, in the 1920's they didn't yet have a tradition of settling internal party differences through violence. That came later as Stalin gained more absolute power in the 1930's. So, how could a civil war have happened? Well, the best starting point I can find is early 1926. 

The Soviet Union in early 1926--Politics: By 1926, Stalin had essentially won the Trotsky/Stalin rivalry. Trotsky still considered himself a factor, but he wasn't really. Power inside the Communist party had shifted to two factions of the old Bolsheviks, with Stalin maneuvering between them to accumulate power for himself. 

The "left-wing" of the old Bolsheviks had a stronghold in Leningrad. They felt that the Soviet Union should squeeze the peasants to accumulate the resources for rapid industrialization--the policy that Stalin eventually followed after he eliminated the people who advocated it. By 1926 Trotsky was gravitating toward the left-wing faction, though he wasn't really a part of it, and had feuded bitterly with some of its members earlier. 

The "right-wing" of the old Bolsheviks felt that the Soviet Union should industrialize by continuing the New Economic Plan (NEP), a carefully controlled flirtation with capitalism that Lenin had brought in as a temporary measure to get the Soviet economy going again after the Russian Civil War of 1918-21. In our time-line, Stalin initially sided with the "right-wing" and helped them push the left out of power within the party. He then turned on the right, adopting the policies of the left and pushing the right-wing out of power within the party.   The left-wing had lost their base in Leningrad by late 1926, so the POD needs to be before then. 

The Point of Divergence: Attempted assassination of Stalin in early 1926. We'll decide later if the assassination attempt is successful. First we have to figure out a plausible assassin. Trotsky and the old Bolsheviks can probably be ruled out. They didn't think that way until it was too late. Also, Stalin was pretty paranoid. He would probably detect and foil an attack from that source. People totally outside the Communist party would probably not have enough information to carry out such an attack. 

I could see two plausible avenues for such an attempt. The first would be internal: someone close to Stalin realizes what the man is and impulsively takes action. Or one of his hatchetmen gets paranoid and decides to go after Stalin before Stalin goes after him. The second would be from the Communist party of Soviet Georgia. Stalin was from Soviet Georgia and was raised in a Georgian tradition of blood feuds. That's part of the reason he was so capable of acting in a way that the other old Bolsheviks couldn't cope with. 

There were other Georgian Communists from the same blood feud background though--some of whom had locked horns with Stalin already. Let's say one of them realizes that he is a dead man if Stalin continues to accumulate power. He strikes first.

 Neither of those scenarios seem all that probable, but they are not impossible. Stalin was riding a tiger in attempting to seize absolute power. One slip and he would have been dead. I'll assume for the rest of the scenario that an assassin gets close enough to have a realistic chance of killing Stalin and maybe some of his allies. I'll decide later if we want him to die of the wounds. For now we'll have him paralyzed from the waist down and in the hospital fighting for his life. Several of his allies on the Politburo and in the Central Committee die, along with the assassin. That cuts out the center of the Communist party and leaves the left and right wings deeply suspicious of one another. 

The balance of power on the Politburo is up in the air. The situation is made more complicated by the fact that Stalin has been gradually developing a power center outside the formal structure of the Communist party. This power center is composed of hatchet-men--in many cases essentially criminals. This faction has no future anywhere near the center of power if either the left or the right wing of the Communist party wins out, or if the two wings settle their differences without violence. Stalin's hatchet-men do, however, have a future if the two factions resort to violence to settle their differences.

 Mutual suspicion is already there. If some of the hatchet-men develop it a bit more with actions of their own, then the Communist party could split along regional lines, with the left-wing centered around Leningrad and the right centered around Moscow. Each side then appoints its own list of people for the rest of the Politburo and declares the other side's members expelled from the party. The two sides come to blows in towns and cities where the division of loyalty is fairly even.

The conflict spreads to the non-Russian Soviet Socialist Republics like the Ukraine and Byelorussia. There, nationalist factions of the local communist parties tend to initially side with the Moscow faction, while pro-Russian members tend to side with the Leningrad faction.

The leadership of the Red Army tries desperately to keep the army out of the conflict, but that proves impossible as the violence between the factions becomes more intense. The army splinters along ethnic lines in parts of the Ukraine and Byelorussia.

 The Leningrad faction elevates Trotsky to a nominal position as head of state, and he pushes for the army to "defend the revolution". He played a major role in building that army, and still has quite a bit of support. The war of words intensifies, with the Leningrad faction accusing the Moscow faction of betraying the revolution to the capitalists and the Moscow faction accusing the Leningrad faction of complicity in the assassination attempt.

 The secret police also split, with bitter battles in the peripheral areas, and a complex series of betrayals and assassinations in the major cities. The secret police have nowhere near the power in 1926 that they did in the 1930's though and the organization splinters on approximately the same lines as the party. There are dissident factions in both Leningrad and Moscow though, and their activities make both cities war zones for almost a month in the summer of 1926. Several additional prominent Communists on both sides are killed in the civil war, and both sides start ruthless purges of the opposition where they are strong enough to do so.

 Large parts of the countryside are still able to stay out of the fighting for a while, as regional authorities try to keep a lid on things or figure out which side is likely to win. There are pockets of fighting throughout the Soviet Union though, and they tend to spread as the two factions try to gain control. 

As the fighting escalates, various neighboring countries begin showing an interest in seizing peripheral areas of the Soviet Union. Japan and Poland keep a close eye on the situation, but for now they decide to wait for future developments.

 The communist parties in the west start to split too, with pro-Leningrad and pro-Moscow factions forming in some countries, while others side almost completely with one faction or the other. A few communist parties are able to remain neutral.

 The violence sputters along at a fairly low but gradually increasing level through the summer of 1926. Neither side really wants a full-scale civil war, but distrust grows with the violence. The Moscow faction of old Bolsheviks finds itself much more dependent than it wants to be on Stalin's hatchet-men. Those hatchet-men quietly ratchet up the level of violence and their own power. They also "uncover" pro-Leningrad plots within the Communist party and then within the army around Moscow. Both factions maneuver to replace army leaders with people they can rely on. That makes it impossible for the army to stay out of the conflict. 

By winter of 1926, the army has been sucked into the violence in much of the Soviet Union. Stalin gradually recovers some of his strength by winter, but he is still hospitalized, in a wheelchair, and is unable to play much of a role in government. Both factions pay lip-service to his continued role, but have in reality replaced him. In spite of his weakness, he resumes maneuvering for power in the Moscow faction's government by late 1926.

The course of the war--politics: By fall of 1926, the factions are sorting themselves out in terms of control of the country. The Leningrad faction controls a large area around and to the north of Leningrad. They also control pockets of the Ukraine around some of the cities with large ethnic Russian populations. They control part of the ByeloRussian SSR. Some of the Caucasus SSR's like Georgia and Azerbaijan are allied with the Leningrad faction, although actual control is in the hands of increasingly nationalist local Communists. Several of the SSRs in Soviet Central Asia have sided with the Leningrad faction, mainly as a way to gain increased nationalist autonomy. Ethnic Russians and Jews have been targeted for ethnic violence in several of those states. Several Cossack groups side with the Leningrad faction--more because it is far away and fighting Moscow than for any other reason. Siberia is a mishmash, with local communist parties or army units theoretically siding with one faction or another but many times actually reveling in virtual independence. 

The Moscow faction controls a large part of the core Russian ethnic area. They theoretically control most of the Ukraine and Byelorussia, though the local Communists in those areas increasingly regard themselves as allies rather than subordinates. The Moscow faction would love to do something about that, but they can't afford more enemies at the moment. The Armenian SSR is theoretically controlled by the Moscow faction, as are parts of Siberia and the Soviet Far East. 

Anti-communist groups inside the Soviet Union have taken advantage of the situation to try to grab for power. In some cases, that struggle is outside the framework of the two factions. In some cases anti-communists, especially nationalists within the non-Russian SSRs, try to play to one faction or the other, usually siding with the faction furthest away from their SSR.

 The Course of the War--Military: By fall of 1926, large parts of the Red Army are fully involved in the fighting. The Leningrad faction (I'll call them the Trotskyites though real power lies elsewhere) has several initial advantages. First, they control more of the cutting edge of the army because they are closer to the various western borders of the Soviet Union. Also, parts of the Red Army gravitate toward them in reaction to the activities of Stalin's hatchet-men. Also, the Trotskyites are quicker to mobilize their part of the economy for war. Also, the SSRs that side with the Trotskyite faction genuinely are willing to fight against Moscow, whereas the ones that side with Moscow are mainly concerned with defending their autonomy. 

On the other hand, Moscow controls the bulk of the transportation network. It also controls a larger base of ethnic Russians. As time goes on, it has an advantage in the propaganda war because the Trotskyite faction genuinely does stand for something very much against the interests of the peasants who are still very much a majority in the Soviet Union. 

The sides are evenly enough matched that the war could go on for years. The armies grow in size, but both of them are still primarily foot infantry and horse cavalry, with a few small motorized units and a smattering of tanks and aircraft. Armored trains play a major role. The fighting is not the static type of World War I. The lines are too long and the armies too small and low on firepower for that. It is still very bloody and destructive as it seesaws back and forth across the Russian countryside.

 That destructiveness is amplified as first one side and then the other begins using the primitive but deadly biological weapons that the Soviets have already been researching. Typhus breaks out on both sides, either naturally or through human intervention. Both sides are aware of the biological weapons programs, and the advent of typhus stimulates their paranoia, making the world's first biological war almost impossible to stop once it gets started. 

Outside military observers are very interested in the fighting, especially the Soviets' potential adversaries like Japan and Poland. The Soviets are not particularly forthcoming in terms of information. The clandestine German training program in the Soviet Union is engulfed in the chaos, and the Germans are forced to flee, leaving their equipment behind, and dealing German clandestine rearmament a damaging blow.

Stalin recovers enough to start playing a role in politics again in early 1929. Though still paralyzed, he pushes himself back into a position of power. He pushes the Moscow faction to squeeze the peasants like the Leningrad faction is doing, though in more subtle ways. 

The peasants of the Moscow faction's area have had a chance to hope for a better future for the last several years though, and they raise a massive peasant revolt, which spreads to the other side of the line between the two factions. In the summer of 1929, the two communist factions almost stop fighting as they join in a tacit alliance to put down the peasants. The peasant revolt lasts until the fall of 1929, then sputters without strong leadership, as both sides combine displays of ruthlessness with temporary concessions. 

The two Bolshevik factions then turn on one another again. Stalin has been rapidly regaining power in the Moscow faction's area. He begins moving in on Red Army decisions. That results in disaster--loss of over a hundred thousand men and a considerable amount of equipment in February of 1930. Stalin is a very poor wartime commander, although in our time-line he eventually realized that and gave the professionals enough latitude to win World War II for him.

 In this time-line, the defeat is the last straw for the old Bolsheviks. They attempt to remove Stalin. He strikes back through his hatchet-men. A civil war within a civil war breaks out. The bulk of the army sides with the Old Bolsheviks. That helps, but the army is not good at the kind of war that Stalin fights, with his hatchet-men striking at key opposition leaders without warning. A series of bombings kills off several key Old Bolshevik and army leaders. The survivors ally with Stalin's old rivals from Soviet Georgia and strike back. 

The Leningrad faction takes advantage of the confusion to launch a major offensive. A faction of the Moscow faction's army is disgusted by the fighting and attempts a coup in conjunction with that offensive in order to end the war once and for all. Unfortunately for them the Leningrad faction's offensive bogs down in the mud of the spring thaw.

A vicious, destructive, three-cornered war goes on for the next couple of months in Moscow. By May, Stalin appears to be gaining the upper hand, but he rules over a ruined city, and large parts of his faction's army deserts, while more is ready to if it gets a chance. His rule doesn't extend much beyond Moscow and a few other nearby cities. 

The peasants have taken advantage of the chaos to revolt again. Parts of the Moscow old Bolshevik faction have fled to the Soviet Far East, where they establish a rival regime. The Russian heartland is a patchwork of Stalinist enclaves, areas held by various peasant factions, and areas held by local military units in nominal alliance with one faction of another, or in a few cases even by non-Bolshevik worker cooperatives. 

In this chaotic situation, various exile groups see an opportunity. Mahkno, the old Ukrainian anarchist goes back to his old area in the South Ukraine and attempts to raise a revolt. The Ukrainian Bolsheviks have managed to position themselves as protectors of the peasants against the demands of the warring factions though, and his revolt fizzles. He escapes the collapse and heads northeast to Ukrainian-speaking enclaves in the Russian republic, where he is more successful, raising a moderate-sized army. 

Various exiled Cossack leaders, die-hard tsarists, and 'white Russians' from Manchuria all try to take a hand in the fighting. The tsarists actually do relatively well for a while, as peasants remember the relative calm of the Tsarist reign with an unrealistic nostalgia. 

In the summer of 1930, the Leningrad faction advances into this anarchy. By October they have Moscow surrounded and are moving past it. The fall muddy season stops their advance, but they spend the next several months besieging what is left of Moscow. 

Stalin has no place to go. He is still wheelchair bound and extremely paranoid even by his standards. Moscow becomes a place of death as he gives full rein to that paranoia, sending his loyalists to round up people on the slightest suspicion of disloyalty. He does manage to motivate the remainder of Moscow's citizens to a fanatical resistance that forces the Leningrad faction to take the city one block at a time.

By January 1931, Moscow can no longer resist. Stalin and his loyalists set off explosives to destroy the remaining infrastructure of the city, then attempt to escape in the confusion. Many of them make it. Stalin doesn't. He is killed in mid-January by a stray bullet. 

The major part of the war appears over, but the cleanup continues into 1931. By the end of 1930, Japan has grabbed some territory in the far east. The various Soviet Socialist Republics have gathered a lot of autonomy and most have armed forces to defend that. 

In 1931 the Leningrad faction has to make some choices. They want to finish consolidating absolute power inside Russia. They also want to eject the Japanese. They also needs to regain control of the non-Russian republics inside the Soviet Union. Non-communist rebels are still in control of large parts of Soviet Central Asia. The Communist parties are still in control in the Ukraine and Byelorussia, but Nationalistic Communists now dominate, and opposition parties are tolerated to some extent. 

The Russian base of the Communist regime is absolutely devastated, in worse shape than it was at the end of the first Russian Civil War. The death toll of over four years of extremely ruthless war, starvation, and disease is very high—in the ten to fifteen million range. And the war isn't over yet, nor have the diseases been brought totally under control. 

The two factions have both generated some new industrialization within their enclaves, though both sides were short of oil so the industrialization tends to be more coal-based than in our time-line. It was also on a rather small scale, and much of the Moscow faction's part of it has been destroyed by the fighting. Neither side had the resources to engage in large-scale industrialization while still keeping their armies supplied. Both sides emphasized artillery and rifle production rather than tank and aircraft production, though both sides did develop large though primitive tank and aircraft parks. 

The Leningrad faction has come to pay lip-service to a very high degree of autonomy for the Ukraine and Byelorussia. During the war they had to do that to keep those Soviet Republics from participating whole-heartedly on the other side. Now they have to decide whether or not to continue doing it. During the war, Poland gradually asserted a kind of protectorate over a semi-independent Byelorussia--mobilizing at one point to keep Trotskyite forces from coming too close to the Polish border. That has to be a consideration. Also, both Byelorussia and the Ukraine have managed to escape the civil war comparatively unscathed and with a considerable amount of autonomous military power. 

The Leningrad faction itself has the potential to splinter. Trotsky considers himself the architect of its victory, while the other members of the faction have been quietly maneuvering to limit his power. The Communist party has lost a great deal of hard-won legitimacy as a result of the fighting. It has also spent essentially every bit of gold and foreign exchange that it has access to. 

The foreign communists have been split by the fighting and have lost droves of talented people, as those people become disillusioned by the fighting. Many of those people are still looking for an alternative to depression-era western capitalism, and they become anarchists or fascists, or join one of the radical labor-based movements. 

This is getting rather long. What do you think so far? Where do you think this scenario would go from here? Comments are very welcome.