Tango Province
In 1600 at the Battle of Sekigahara, Chikamasa was part of Ishida Mitsunari's force while his son, Ikoma Kazumasa was part of Tokugawa Ieyasu's force. Chikamasa remained at Sanuki and sent his retainers in his stead to attack Tanabe castle in Tango Province. He chose this strategy to preserve the Ikoma clan regardless of which side prevailed. Because Kazumasa fought in Ieyasu's force, Ieyasu allowed Chikamasa to rule his existing domain after the battle, but Chikamasa took responsibility for fighting on the losing side by transferring the headship of the family to Kazumasa.