In 1600, he took sides with the Tokugawa Ieyasu Eastern army in the Battle of Sekigahara.

[2] However, historians arguing that the reason were because Sadatsugu behaved suspiciously visiting Toyotomi Hideyori at Osaka Castle, without the Shogunate approval, while the land which Sadatsugu occupied was considered as important military strategic location.

[3] Furthermore, it was more though that in Ieyasu perspective to strip the land and give it to the Todo clan as political strategy against the Toyotomi clan, as despite his patronage to the Toyotomi family, Tōdō Takatora were considered as a close ally of Ieyasu.

Thus by making him to control the portions of Iga province, it could pushed more strategic locations to the influence of Shogunate without directly provoking the Toyotomi faction in Osaka.

However, his son, Tsutsui Juntei was killed in action during Summer Siege of Osaka.