At the Battle of Sekigahara in 1600, Tadakatsu was part of Ieyasu's main troop and engaged in an operation to send letters to territorial lords to have them take sides with the Eastern Army.
慶長5年（1600年）の関ヶ原の戦いでは家康本軍に従軍し、諸大名に書状を送って東軍方につける工作にも活躍した。