In May 1575, Katsuyori TAKEDA attacked Nagashino-jo Castle, the residence of Sadamasa OKUDAIRA, leading 15,000 troops, in order to kill him who betrayed the Takeda clan and became a vassal of the Tokugawa clan after the death of Shingen., i.e., Katsuyori's father.
天正3年（1575年）4月、武田勝頼は父・信玄の死後、武田氏より離反し徳川氏の家臣となった奥平信昌を討つため、1万5,000人の軍勢を率いて貞昌の居城・長篠城に攻め寄せた。