Kōsaka Masanobu

Kōsaka Masanobu (1527-1578) is a renowned Sengoku-period warrior and a commander of great strategic skill, counted among the Four Great Retainers who served Takeda Shingen. His real name was Kasuga Toratsuna, and he was also known by the common name Kōsaka Danjō. Even after Takeda Shingen's death, he continued to support Shingen's heir, Katsuyori, but he died of illness in Tenshō 6 (1578), three years after the Battle of Nagashino.

According to the "Kōyō Gunkan," Masanobu was born in Isawa in Kai Province (present-day Yamanashi Prefecture), the son of a farmer named Kasuga Ōsumi, and he entered the service of Takeda Shingen from a young age. He rose to prominence as one of Shingen's attendants and eventually grew into a senior retainer responsible for the Takeda army's northern Shinano front. In particular, in the Battles of Kawanakajima (five engagements between 1553 and 1564), he fought fiercely against the Echigo forces led by Uesugi Kenshin, and his strategic skill was fully displayed. Among them, in the Fourth Battle of Kawanakajima (1561), according to the "Kōyō Gunkan," while the Takeda army carried out the woodpecker tactic, Masanobu joined the detachment that attacked the Uesugi forces on Saijō-yama and distinguished himself.

One of Masanobu's most important roles was to confront Uesugi Kenshin for many years as the castellan of Kaizu Castle (present-day Matsushiro, Nagano City, Nagano Prefecture) in northern Shinano. Kaizu Castle was the linchpin of the Takeda clan's control of northern Shinano, and Masanobu, as its castellan, defended it for many years. Its defense was so firm that even Uesugi Kenshin could not easily take it. Having earned Shingen's deep trust, Masanobu is also said to have been one of the authors (or original compilers) of the "Kōyō Gunkan," and he played an important role in passing down the Takeda clan's military laws and tactics to later generations.

Known for his wisdom and his skill in both the military and literary arts among the Four Great Retainers of Takeda, Masanobu's life embodies the ideal image of a Sengoku warrior.