Shingen, who was also concerned that the forces of Nobunaga had expanded, carried out a large-scale invasion of the Provinces of Totomi and Mikawa to subjugate Ieyasu TOKUGAWA who was a sworn ally.
信玄も信長の勢力拡大を危惧したため、元亀2年（1571年）2月、信長の盟友である徳川家康を討つべく、大規模な遠江・三河侵攻を行う。