There's one thing about the book that still puzzle me. After meeting with all the Heads of Families in the USA, Don Corleone go back home to have a meeting with his Caporegimes and Tom Hagen. Then the two Caporegimes leave and Hagen stays alone with Don Corleone. To take the direct quote from the book, I still wonder what was that "one dark fact" no one mentionned.

Direct quote from the paper back copy on page 297 Book V:
"But now he(Hagen)was sure that in the subtle and complex mind of Don Corleone a far-ranging plan of action was being initiated that made the day's happennings no more than a tactical retreat. And there was that one dark fact no one mentioned, that he(Hagen)himself had not dared to ask, that Don Corleone ignored. All pointed to a day of reckoning in the future."

Can anyone can clarify my mind on what was implied by that "one dark fact"?