In 1589, Masamune designated himself as Aizu shugo (provincial constable) and won a fatal battle against the Ashina clan, a ruler of the southern Oshu region, which made the Date clan to achieve the largest territory in its history.
1589年には、会津守護を称して南奥州の覇者であった蘆名氏と雌雄を決し、これを滅ぼして伊達氏の領土は最大となった。