Tenbun no ran
The Tenbun no ran was a feud between Date Tanemune and his son, Date Harumune when Tanemune attempted to adopt another of his sons, Date Sanemoto, to the Uesugi clan which Harumune steadfastly opposed. What was initially a disagreement eventually escalated into a small internal clan war. Though Tanemune had the advantage for some time, Harumune gained the support of the Soma and Asihina clans, turning the feud in his favor. Eventually, Tanemune lost and was forced to retire in 1548, allowing Harumune to take control of the clan.
The Tenbun no ran lasted roughly 6 years, beginning in 1542, causing much strife within the Date clan and many of its retainers, leaving it weak and unstable. The Date clan would not restabilize until Harumune's son, Date Terumune would take over, who in turn retired in favor of his son, Date Masamune in 1584.