Feud

A feud is a long-running argument between two groups of people. Feuds are noted for having no particular ending or beginning - generally both sides cite 'originating events, attacks and reprisals. Because the validity of the arguments are lost in time the feud has no natural end, continuing until either one side is crushed, some external event forces the two sides to reconcile, or a paragon from one side convinces first their own people to turn away from the feud and then the other side.
Feuds are often intermittent as key members of one side or another are killed. Often the feud is continued at the urgings of inactive members of both sides who urge more active members to fight for the families pride.
Feuds are common between noble families, rival nations, and other well-matched groups that compete for similar resources. Disputes between unmatched sides rarely evolve into feuds as the greater side wins, disputes between dis-interested parties do not become feuds as they lack any cause to keep the parties in conflict. This makes feuds very hard to solve - not only does both side have multiple overlapping grievances (often justified on both sides) but they are also fundamentally in opposition on some point - access to resources, the drive to dominate an area, etc.