Opposite sides

• Robin Hood and the Sheriff of Nottingham, e.g
• "Wise men learn much from ___": Aristophanes
• Parties far from festive
• Exes, sometimes
• Ones at war
• Napoleon and Wellington
• Members of the Legion of Doom, to members of the Justice League
• Brutus and Antony, e.g.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Enemy \En"e*my\, n.; pl. Enemies . [OF. enemi, F. ennemi, from L. inimicus; in- (negative) + amicus friend. See Amicable .] One hostile to another; one who hates, and desires or attempts the injury of, another; a foe; an adversary; as, an enemy of or to ...

Wiktionary Word definitions in Wiktionary
n. (enemy English)