There was a part of this movie that I did not understand until much later. When the senior Godfather Vito Corleone called the meeting of the Five Families at that conference, he swore on the soul of his grand-children that he would not be the one to break the peace which they had made there, on that day. He forgoes the vengeance of his son, and he keeps his oath.
But we know from other sources that his vengeance is actually coming. That is because there was an omission in his vow. That oath was not incumbent upon his heir and successor, Michael Corleone. He said nothing at all about MICHAEL not breaking that peace. So what happens? Vito dies, Michael takes over as the new Godfather, and wreaks his vengeance on everybody: A real blood-bath! Am I correct?