Twenty or thirty pages of information handed to any of the major world powers around the year 1925 would have been sufficient to chang the course of world history. 

虚拟语气的条件不一定出现，或者不一定用 if 从句显示。这叫做虚拟语气的“含蓄条件”。其实，本句作定语的handed，就可以扩展为一个“虚拟条件句”：

Twenty or thirty pages of information handed to any of the major world powers around the year 1925 would have been sufficient to chang the course of world history.  等于：

Twenty or thirty pages of information, if they had been handed to any of the major world powers around the year 1925,  would have been sufficient to chang the course of world history.