For a revolution to succeed, the one and only thing that matters is that the historic forces are behind it.

Charismatic leaders, populist support etc... these things are little more than catalysts when you really look into things. They let the historic forces flow more freely, they are in themselves largely a product of them, but they are never a significant factor in causing the revolution themselves.

All those 'turning points' in history were nothing unexpected and never brought any real changes, they were the inevitable culmination of all the events that proceeded them (the Arab empire had reached its natural extent by Poitiers anyway). All those radical ideologies were a product of their earlier counterparts that formed their roots (even Marxism is heavily influenced by the likes of Locke, he just applied it to a different time in history). All those great events such as national unification were really nothing more than the consolidation of the changes that had already taken place (the German people were sharing an identity well before Germany came into being).

There is no significant unexpected event in history, everything is inevitable!