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In 1563, Katsumasa became head of the clan, when his father Nagamasa died. Prior that time, the Ikeda clan were vassals of the Miyoshi clan. However, when Miyoshi Nagayoshi died the Miyoshi clan became weak, and retainers of the Miyoshi clan were divided. Nagamasa remained loyal to the Miyoshi clan and battled against Itami Chikaoki, who allied with Matsunaga Hisahide.
In 1568, when Oda Nobunaga marched his armies into Kyoto in support of Ashikaga Yoshiaki, Ikeda Katsumasa battled against him, as part of the Miyoshi clan. Katsumasa lost the battle and surrendered. As a result, he served Nobunaga.
Nobunaga let Katsumasa, Chikaoki and Wada Koremasa rule Settsu Province; he thought if he let powerful clans such as the Ikeda clan become his retainers, they would rule with stability. They were known as the Three Shugo of Settsu (摂津の三守護 Settsu no San Shugo?).