A feud (pronounced /ˈfjuːd/) (referred to in more extreme cases as a blood feud or vendetta) is a long-running argument or fight between parties—often, through guilt by association, groups of people, especially families or clans. Feuds begin because one party (correctly or incorrectly) perceives itself to have been attacked, insulted or wronged by another. Intense feelings of resentment trigger the initial retaliation, which causes the other party to feel equally aggrieved and vengeful. The dispute is subsequently fuelled by a long-running cycle of retaliatory violence. This continual cycle of provocation and retaliation makes it extremely difficult to end the feud peacefully. Feuds frequently involve the original parties' family members and/or associates, and can last for generations.
Up to the early modern period, feuds were considered legitimate legal instruments and were regulated to some degree. Once states asserted and enforced a monopoly on legitimate use of force, feuds became illegal and the concept acquired its current negative connotation.