1342年（康永元年/興国3年）征西宮懐良親王が谷山城に入城して南朝方に勢いがつき、貞久は度々谷山に攻撃を掛けたが落とせず、戦況はなかなか有利にならなかった。

The Southern Court gained momentum as Imperial Prince Kaneyoshi of Nishinomiya entered the Taniyama-jo Castle in 1342, and the war situation did not improve easily, for Sadahisa's repeated attacks on the Taniyama-jo Castle proved to be futile. - Wikipedia日英京都関連文書対訳コーパス

菊池武光、赤星武貫、宇都宮貞久、草野永幸ら南朝方は正平14年/延文4年（1359年）の筑後川の戦い（大保原の戦い、福岡県小郡市）でこれを破り、正平16年（1361年）には九州の拠点である大宰府を制圧する。

The southern court forces, including Takemitsu KIKUCHI, Taketsura AKABOSHI, Sadahisa UTSUNOMIYA, Nagayuki KUSANO, defeated Yorinao SHONI in the Battle of Chikugo-gawa River (the Battle of Ohobaru, Ogori City, Fukuoka Prefecture) in 1359, and conquered Dazaifu, a stronghold of Kyushu in 1361. - Wikipedia日英京都関連文書対訳コーパス

南朝勢力を強化するために、後醍醐天皇の皇子である懐良親王が征西将軍として派遣され、筑後川の戦い（大保原の戦い）では、南朝方の懐良親王、菊池武光、赤星武貫、宇都宮貞久、草野永幸らと北朝方の少弐頼尚、少弐直資の父子、大友氏時、宇都宮冬綱ら両軍合わせて約10万人が戦ったとされる。

In order to bolster the Southern Court's military might in Kyushu, Emperor Godaigo's son Imperial Prince Kaneyoshi was dispatched there as Seisei shogun (General of the West), and in the battle of the Chikugo river (also known as the battle of Ohobaru) over 100,000 men were said to have fought, including Imperial Prince Kaneyoshi, Takemitsu KIKUCHI, Taketsura AKABOSHI, Sadahisa UTSUNOMIYA, and Nagayuki KUSANO for the Southern Court, and father and son Yorihisa and Naosuke SHONI as well as Ujitoki OUCHI and Fuyutsuna UTSUNOMIYA for the Northern Court. - Wikipedia日英京都関連文書対訳コーパス