Left SR don't revolt against the Brest-Litovsk Treaty

The Left SR were one of the few Left Wing groups the Bolesheviks invites to join their Government following the overthrow of the Provisional Government in 1917. They were given some measure of responsibly, with 4 Commissar positions and influence in the Cheka.

However they were bitterly opposed to the Treaty of Brest-Litovsk and would split with the Bolesheviks over the peace, before trying to force a general uprising in July 1918 against it via the assassination of the German ambassador to Russia and a brief uprising within Moscow itself.

This failed and led to the general suppression of the Left SRs (though a number of the rannk and file joined the Bolesheviks), though these events helped lead to Danny Kaplan's attempt on Lenin's life the next month.

So what if the SR leadership decide that a general revolt isn't possible and try to live with the peace, instead trying to continue to influence the government. I personally think the numbers were against them and that the Bolesheviks would have eventually forced them out of power. However avoiding the uprising (and the assassination attempt on Lenin) provide major butterflies of their own.
 
(1) Brest-Litovsk was only the immediate occasion for the breach between the Bolsheviks and Left SR's, not the only reason. For example, the Left SR's were opposed to the Bolshevik's setting the poor peasants against the rest of the peasantry (theoretically just the "kulaks" but in practice much of the middle peasantry as well) with the Committees of the Rural Poor.

(2) The Left SR withdrawal from Sovnarkom has been criticized as removing an obstacle to one-party dictatorship by the Bolsheviks, but it boosted the Left SRs' popularity immensely. In the spring of 1918, all sorts of opponents of the Bolsheviks and the treaty--Left SR, SR, and Menshevik--made big gains in elections to the soviets. (I tend to think this was more due to the dissatisfaction with the economic conditions than with opposition to the treaty itself, which was more widespread among the middle class than among the peasants, who could only welcome an end to the war--but who didn't want a new war in the village between the poor and the better-off peasants.) The Left SRs with their heavy peasant support (in what was after all a predominantly peasant country) thought they could achieve a majority at the Fifth Congress of Soviets that would assemble on July 6, 1918. Between April and the end of June, membership of the party went from 60,000 to 100,000.

(3) Lenin, however, frustrated the Left SRs' plans by having Bolshevik-organized "Committees of Poor Peasants" proclaim that they had the right to represent at the Congress all those districts where local soviets had not been "cleansed of kulak elements and had not delivered the amount of food laid down in the requisitioning lists of the Committee of Poor Peasants." (Quoted in Geoffrey Swain, *The Origins of the Russian Civil War*, p. 176) This blatant manipulation assured the Bolsheviks of a majority at the Fifth Congress of Soviets.

(4) The Left SR reversion to methods of terror--the assassination of Mirbach and later the "uprising" (which was arguably a defensive move by the Left SR's after their delegation to the Congress of Soviets was placed under arrest)--has to be understood in the context of the Bolsheviks depriving the Left SR's of any chance of getting a majority by democratic means.

It just seems to me very unlikely that the Bolsheviks could tolerate the Left SR's indefinitely, Brest-Litovsk or no Brest-Litovsk. They were determined on policies (grain requisitioning, the Committees of Poor Peasants, etc.) that were too unpopular to allow another party to continue to exist and gain an advantage from the grievances these policies caused.
 
Fair enough, I admit I thought this was an unlikely scenario, for all the reasons you mentioned above. However if we put the split off for a month or two (say by no consensus in the leadership on a course of action) would that have any material difference? Even butterflying Lenin's injuries should Kaplan's attempts not happen (or a different fail) would be a major PoD of its own.
 
Brest-Litovsk was certainly the central point in which the split occurred, even if there were other things the two parties disagreed upon. The Left-SRs helped shape a lot of early Soviet policy, including in regards to the peasants and agriculture, and were generally a moderating influence on Bolshevik positions. The Bolsheviks themselves were largely divided on several issues and the Left-SRs utilised these divisions to ensure their voice was heard. If they'd remained within government they could have continued in their role of influencing policy and moderating the more radical Bolsheviks. They were a coalition and if modern political coalitions are anything to go by it's often the case that the weaker organisations have a lesser role but nonetheless can have a great opportunity to shape the politics of the era.

According to Lara Douds: "It is clear that that there was only one reason for the Left S.R. party’s withdrawal from the central Sovnarkom: the Fourth Congress of Soviets had adopted a resolution, the ratification of the Brest-Litovsk treaty, that the Left S.R.s were convinced had undermined the October Revolution. The Left S.R.s, if they allowed their representatives to participate in Sovnarkom, would be collaborating in policies they were sure would lead to the smothering of the revolution. This did not mean that they were ready to abandon the Bolsheviks completely: ‘as far as Sovnarkom brings to life the programme of the October Revolution, the party promises its support and assistance’. Their resignation statement confirmed that when the Bolshevik party returned to the revolutionary path from which it had strayed, the two parties would collaborate in government again.
The Left S.R.s had been correct to surmise in November 1917 that they could only effectively blunt the coercive, centralizing and anti-peasant bias of the Bolshevik party from inside the government itself. Programmatic differences with the Bolsheviks quickly widened after the Left S.R. departure from Sovnarkom and any moderating influence they had had quickly dissipated. To Leninist Bolsheviks the solution to the continuing disintegration of economic and political life, and to the threats posed by foreign and domestic enemies, increasingly lay in dictatorship, institutional centralization, and the utilization of bourgeois specialists. To the Left S.R.s, and moderate Bolsheviks such as the Left Communists, who were committed to the ideal of worker and peasant empowerment exercised through democratic soviets, these policies were very distasteful, as Karelin made clear to the C.E.C. on 29 April when he challenged Lenin’s fundamental assumptions and policies.102 After the Left S.R.’s resignation from Sovnarkom, its policy towards the countryside also began to change direction, and the tendency towards the use of force to solve the urgent urban hunger crisis grew." - ‘The dictatorship of the democracy’? The Council of People’s Commissars as Bolshevik-Left Socialist Revolutionary coalition government, December 1917–March 1918 by Lara Douds

In the first paragraph she's quoting the official minutes of the party during this period. The party still remained committed to the ideals of the revolution but were, ultimately, unable to accept the Brest-Litovsk treaty. They hoped that their dramatic action would encourage the left communists to join them and end the peace talks but instead it isolated the left communists and gave credence to the less moderate aspects of the Bolsheviks. It could even be said that Brest-Litovsk drove the so-called 'right' SRs further into the camp of the Whites as well.
 
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