Interesting question, although I would retreat to the principle that seldom is there "one factor" or "one cause" that can primarily explain any complex phenomenon. But in the spirit of your question my best answer is that the most pivotal event is when any one individual realizes the difference between choosing to follow their reason wherever it may lead or instead choosing to follow their irrationalities for the sake of cherished myth, comforting answers, or relief from complexity. That is the moment that is most pivotal. The conditions of our civilization depend to a very large degree on the decision that every individual makes when they confront that choice. And we each confront it quite often.
As to the landmark events in world history - my opinion is that there is not "one" any more than one snowflake best explains why my roof collapsed.
Thank you for your question.
Steve Hindes