During the reign of his father, Yoshitsuna, Ouchi clan complied with the demand of the Tamura clan and banished their lord, Hisayoshi ISHIBASHI; and Yoshitsuna became the feudal lord of Shionomatsu (eastern part of Adachi District) and belonged under the command of the Tamura clan; however, Sadatsuna, who took over as head of the family, gradually planned to gain independence due to his frustration about decisions made over conflicts between vassals of the Tamura and Ouchi clans.
大内氏は、父・義綱の代に田村氏の内応工作に応じて主君・石橋尚義を追放し、塩松領主となって田村氏の旗下に属していたが、家督を継いだ定綱は、田村・大内両家の家臣同士の争いの裁決に対する不満から、次第に田村氏からの独立を目論むようになる。