In 1600, he battled Uesugi Kagekatsu, an enemy of Tokugawa's, alongside Date Masamune (his nephew), another lord of the far north. Mogami and Date supported Ieyasu at the famous Sekigahara campaign, he aided in Date's siege of Shiroishi, and was then attacked in his own home castle of Hataya, Kaminoyama and Hasedō. After which Mogami's domain was expanded to 520,000 "koku" in return for his loyal service. This made the Yamagata domain the fifth largest in Japan at the time, excluding the land held by Tokugawa.