I haven't read Belloc's book, but since no one else is answering, let me try to help based on what I know of Napoleon.
The quote you cite basically says that the same drives and qualities that make men great also make them vulnerable to failure -- what makes them great also undermines them.
Napoleon's greatness and his fall seem to me to come from his daring and arrogance. Both of these led him to do things that others would not have attempted. Early on, that allowed him to rule France and conquer much of Europe. But then those same impulses made him overreach and invade Russia. This led to his fall.
So, Napoleon was driven to the top by the same qualities that made him make a fatal mistake.