Though Mitsunari had ruled over the Ise region since August, he made a decision to fight in the fields of Sekigahara due to Ieyasu's quick inverting attack to the west.
8月からは伊勢方面の平定に務めたが、家康ら東軍の反転西上が予想以上に早かったため、三成は関ヶ原で野戦を挑むことを決める。