It is considered that Ieyasu gave this "Assurance of Hyakumangoku" (Hyakumangoku meaning 1,000,000 koku of land) to urge the participation of Masamune to his Eastern Army, because his participation to the Eastern Army was critical in order to hold Kagekatsu UESUGI to Aizu region.
これは、家康が上杉景勝を会津に釘付けにしておくため、政宗の東軍参加が是非とも必要であったことから、100万石のお墨付きを与え、東軍参加を促したとされる。