The problem i see with Trotzky leading the USSR isn't that he would have underestimated fascism, it's that he was really awful at getting the party on his side. I mean, sure, in a scenario where he was in charge during WW2, that obviously would not apply for some reason, but that brings us to the core of the problem with such scenarios: A Trotzky leading the Soviet Union wouldn't be the historical Trotzky, it would be some other dude looking just like Trotzky.
It's my impression that all of the Bolsheviks knew how dire the threat of Nazism was, I mean Trotsky was writing about it all through the 30s.
The problem i see with Trotzky leading the USSR isn't that he would have underestimated fascism, it's that he was really awful at getting the party on his side. I mean, sure, in a scenario where he was in charge during WW2, that obviously would not apply for some reason, but that brings us to the core of the problem with such scenarios: A Trotzky leading the Soviet Union wouldn't be the historical Trotzky, it would be some other dude looking just like Trotzky.