https://en.wikipedia.org/wiki/Speech_act https://en.wikipedia.org/wiki/Performative_utterance
Was it after they seized power? They were so appreciative of free speech, that they immediately repressed it.
Alternatively maybe it could have been stopped by preventing the industrial revolution. Or by preventing Germany and Italy from uniting as countries at the end of the 1800's/early 1900's. Or by preventing banking interests from lending to industrialists. Or by preventing the Viking invasions. Or preventing the collapse of international papal authority. Or by preventing the rise of papal authority in the first place. Or giving the Chinese first dibs on East Asia. Or giving native Africans gunpowder. Or by any number of other means.
Point being, you seem to think 1)Fascism in Europe during the second world war is the worst event to happen to mankind ever and attempting to prevent it re-occurring justifies any means, even those with potentially worse outcomes. And 2)Fascism arose strictly because some guys popped in out of nowhere and started talking.
What about Pol Pot? What about Stalin? What about the church during the Middle Ages? What about the conquests of Islam? What about the massacres in Medieval China? Events very simply don't happen in a vacuum, (in spite of the popular culture/ public school view of history).
The dangers of those who want to prevent free speech because it offends someone far outweigh the dangers of letting the marketplace of ideas decide which ideas are worth considering. I don't find the rise of the alt-right (which I don't support btw) nearly as alarming as I find the number of young people who think offensive speech should be legislated against or banned. Which is truly an authoritarian and disturbing trend.