Possible Clues:

• Some are bitter or sworn
• Exes, sometimes
• 'Wise men learn much from ___': Aristophanes
• They're opposed in war
• Opposing forces
• 'A man cannot be too careful in the choice of his ___': Wilde
• They're not for one
• Opposition mentioned by Connolly, in a book of some promise
• Robin Hood and the Sheriff of Nottingham, e.g
• The Bloods and the Crips, e.g.