The Hatfields and McCoys were two families from the Appalachian Mountains in West Virginia and Kentucky who were involved in a decades-long feud. The term 'Hatfields, to McCoys' is used to describe any long-standing rivalry or OPPOSITION. The crossword solutions for this term are 'FOES' (4 letters) and 'OPPOSITION' (10 letters). Both words accurately describe the relationship between the two families, as they were in constant OPPOSITION to each other.