慶長5年（1600年）9月15日、徳川家康は関ヶ原において石田三成率いる西軍と激突。小早川秀秋の東軍への寝返りが決定打となり、わずか半日で東軍が勝利。この勝利により家康は事実上の天下人となり、慶長8年（1603年）に征夷大将軍に任じられ江戸幕府を開いた。関ヶ原での勝利は家康にとって生涯最大の決戦であり、その勝利なくして江戸260年の平和は実現しなかった。

Hideyoshi's death in 1598 was the underlying cause of the Battle of Sekigahara. Despite constructing the Council of Five Elders and Five Commissioners to preserve Toyotomi rule, his regime fractured within two years. Sekigahara was essentially a power struggle over his succession, with both Eastern and Western armies fighting over his legacy. Hideyoshi's concubine Yodo-dono and their young heir Hideyori served as the symbolic banner of the Western Army.

Ii Naomasa led the 'Ii Red Armour' vanguard and distinguished himself at the forefront of the Eastern Army. During the pursuit of Shimazu Yoshihiro's retreating forces after the main battle, he was shot and seriously wounded. The wound would claim his life the following year, but his merits were recognized, and his son Naitsugu was awarded the large domain of Hikone in Omi. The foundation of the Ii clan's prosperity in Hikone lies in Naomasa's brave fighting at Sekigahara.