In 1399, he provoked a powerful Daimyo in the western region, Yoshihiro OUCHI, and using the excuse that Yoshihiro was taking up arms, he subdued Yoshihiro (War of Oei), and in this way the influential factions that might oppose Yoshimitsu in the western region were eliminated.
1399年（応永6年）には西国の有力大名・大内義弘を挑発し義弘が堺市で挙兵したのを機に討伐し（応永の乱）、西日本で義満に対抗できる勢力は排除された。