Trotsky was a valiant opponent of Stalinism. But he was not someone without political limitations. The Stalinist 'critique' of Trotsky's theory was utter garbage, it totally mangled the understanding of the issues. Trotsky does well to refute the idea that he was anything like Lenin's mortal enemy. But that doesn't mean there are no legitimate criticisms to be made. For the purpose of combatting Stalinist slander Trotsky understandably dilutes the revolutionary position to reduce his differences with Lenin to being merely about conciliationism towards Menshevism. But this dilution doesn't satisfy an understanding of what determined him to not be on Lenin's side. Trotsky's theory was a core part of his political views for which he would not join the Bolsheviks until July 1917. Neither is the history of 'orthodox' Trotskyism a one-sidedly positive one. Organisational questions cannot be separated from political questions.
Much of what is often attributed to Trotsky’s innovation was actually Second-International orthodoxy: ideas of ‘skipping stages’, that the working class must lead, that the Russian revolution could spark a European socialist revolution, and then that this European revolution could allow for socialist development in Russia, were all positions common to Russian Social Democrats leading up to 1905. Kautsky during the 1905 revolution had argued all of these points. Kautsky also argued that the revolution must be led by workers in an alliance with the peasantry, a point to Lenin against the Mensheviks. It was not until the aftermath of 1905 that the Mensheviks explicitly went to the right, though the seeds for such a turn must be seen in their earlier disputes with Lenin.
What made Trotsky distinct was an identification of the democratic and socialist revolutions in Russia: that a provisional revolutionary government dominated by workers would strive to enact socialist transformation *prior to* successful European socialist revolution, *and* that such a situation was a supportable one as the only road by which Social Democracy would not discredit itself. Such an inevitability could only be consummated in successful socialist transformation by the European socialist revolution coming to Russia’s aid. Ultimately, the Russian revolution was doomed to failure unless the European proletariat did their part in the scheme. Trotsky was arguing that the only way Social Democracy could not discredit itself was by instigating a civil war with the peasant majority after taking power, and hope that Europe came to its aid. It’s clear why this was not popular, compared to the idea of winning the peasantry to Social-Democratic leadership, and thereby accomplishing the democratic dictatorship as something immensely significant while still short of socialism. When Social Democrats were trying to convince the peasants that they had nothing to fear from Social Democracy, Trotsky was telling them they had a lot to be afraid of.
Trotsky had it that the Russian Revolution would be spontaneously socialist because the workers' movement in revolution was spontaneously socialist, and the working class would spontaneously achieve a hegemonic position in the revolution. To doubly identify the workers' movement, socialism, and the hegemonic character of the revolution in such a way was ultimately a form of economism. Whereas the SRs thought the revolution would be spontaneously socialist by the character of the peasantry, Trotsky thought the revolution spontaneously socialist by the character of the proletariat. Out of everyone it was Martynov the economist-turned-Menshevik who would agree with Trotsky that a revolution led by the proletariat with the RSDLP in government could not help but be socialist. He just flipped the value judgement to say that such a situation would be terrible and therefore the RSDLP should not participate in a revolutionary government.
The exact character of a worker-peasant alliance was a point of much political difference, nuance, and disagreement. Lenin, while recognising the significance of peasant political formations, did not (unlike Trotsky) think a worker-peasant coalition had to consist of an alliance of parties: it could take a variety of forms of practical alliance and unity in action, a crucial form being the soviet. The point was that the peasants had to constitute a social and political force in their own right, and be recognised as such. Strategic reflexivity to such a force was a key condition for workers achieving revolutionary consciousness as leaders of the ‘people’. What changed leading up to the April Theses was that imperialism made possible a *socialist* revolution of the ‘people’, ‘the whole nation’, the proletariat its vanguard (though the concrete, non-ideal, not purely proletarian situation made for contradictions and roadblocks to be resolved through the international struggle). This is also the logic by which Lenin becomes most enthusiastic about the role of national self-determination in the struggle for socialism. This is the proletariat as the universal class, the leader of all the masses. Economism never understood the significance of having one's sights beyond the workers.
It was on this point that Trotsky ‘underestimated the peasantry’. Trotsky thought that workers would spontaneously come to a leading position in the revolution by fighting for their own interests as a group. This was the premise for workers inevitably bringing on socialist transformation even if the peasants didn’t want it. The peasants were denied the agency of constituting themselves as a coherent political force. Instead the peasants would lump themselves as a disparate mass behind the workers in the democratic revolution, but then turn on the workers when the latter ‘necessarily’ began enacting socialist transformation.
To paraphrase Lenin, organisational questions cannot be separated from political questions. On this, Trotsky's theory corresponded to his political practice, which was to situate himself on the left wing of centrism within the RSDLP. Trotsky’s theory was, despite the differences of policy, more Kautskyan than Lenin and his endorsement of Kautsky’s formal positions. That there was only one inevitable path ‘forward’ for the Russian revolution expressed a unilinear logic of history. It was a kind of narrow focus on the working-class, a Kautsyan theology of the proletariat. It did not sense the potential problem of the Russian revolution being diverted into genuinely ‘forward’ but much less favourable directions by non-Marxist leadership. Lenin was very attuned to the possibility of the revolution being led by the SRs or another political force, the workers being reduced to a non-hegemonic pressure group. This motivated factional struggle. For Trotsky in contrast, the logic of the revolutionary struggle, a teleology towards socialist revolution, would render the Bolshevik-Menshevik division obsolete. Thus Lenin’s supposed dogmatism seemed to be a barrier to getting on with guiding the proletariat along the railroad of fate, rather than a righteous struggle over the character of leadership which Social Democracy would bring to the workers’ movement. It would be not Bolsheviks but some Mensheviks who would briefly play with Trotsky’s theory, and the idea of an exclusively workers’ revolutionary government. Trotsky after 1905 would try to broker a reconciliation of the two factions, earning him the label ‘centrist’ as Lenin described him.
This does not mean that Trotsky's theory was total garbage, or that Lenin was always fully correct at every moment without any development of thought. The idea that Russia would begin the international socialist revolution by itself having a socialist revolution was important. But it was not anything like Trotsky's logic by which Lenin came to his 1917 position. Gramsci criticised Trotsky’s theory in the Prison Notebooks as an anti-strategic proclamation which in the general course of things turned out correct but did not appropriately guide concrete analysis and action. What happened with Lenin was that developments in theories of imperialism and of the state fused with his logic of hegemony. We should reframe our understanding of what happened theoretically in 1917: it was not so much Lenin coming around to Trotsky's theory as it was Trotsky coming around (though arguably not fully) to Lenin's Marxism.