Why were there such powerful generals? Because Rome had decided to become an empire and empires need to expand militarily. Particularly the Roman Empire needed to expand militarily because it always needed new land to give its retired legionnaires. That expansion created the all-powerful general and the incorporation of diverse peoples made it easier for them to be loyal to him rather than to some abstract idea of the republic. Julius Caesar didn't create emperors. Empire created them. Next week we'll be discussing Christianity, so that shouldn't be controversial.