The discussion of late 19th century / early 20th century socialist movements involves numerous characters, technical terms, events, and publications so that having a working knowledge would take quite a time investment. Trotsky mentions the Zimmerwald Conference, which I had never heard of, so this became the starting point. Ostensibly, this was to oppose World War 1, and their conference resulted in the Zimmerwald Manifesto. WW1 was certainly a good starting point, since even, I after looking at the history multiple times, view it as an utterly pointless war for the sake of having a war. There weren't even any implausible pretexts. So here is the evaluation of the cause per the socialists:
"The ruling forces of Capitalist society, in whose hands were the destinies of the nations, the monarchical and the Republican Governments, secret diplomacy, the vast employers' organizations, the middle-class parties, the Capitalist Press, the Church - all these forces must bear the full weight of responsibility for this war, which has been produced by the social order nourishing them and protecting them and which is being carried on for the sake of their interests."
Blame everybody. The workers alone remain untainted in this, which is where the problem lies: How many tradesman were at the Zimmerwald Conference? Almost certainly zero. (Peter the Great was the only Russian tradesman to have achieved power.) And what do the socialists have on offer to replace the list of evil institutions above? Democracy as a means to social unity had already lost its luster due to the challenges of reality. With economic collapse and despair on offer, the socialist apparently just offered up Revolution! as the final answer. Just throw out the bums, and we will give you "hope and change". What this fails to recognize is that whoever ends up in power after the revolution is just the next tyrant, and he will not be able to govern without establishing a new ruling class.
Another elephant in the Zimmerwald room was that of nationalities. The Ukrainians being a typical example. As long as social revolution was in its pre-revolt position, appealing to tribal instincts was in their interest. As soon as power was achieved, the opposite sentiment took hold, culminating in the Ukrainian famine. Which brings us to the point of recognizing that the socialists aren't any different then what went before. They just exploited the gullibility of the uneducated classes.