In the Sengoku period (period of warring states), the Iwamatsu clan who succeeded to the head family of the Nitta clan was defeated by a gekokujo (an inverted social order when the lowly reigned over the elite) by a senior vassal, Yokose clan.
戦国時代になると新田宗家を継承した岩松氏は重臣横瀬氏に下克上される。