Kōsaka Masanobu

Kōsaka Masanobu (1527-1578) was a renowned Sengoku warrior who served Takeda Shingen as one of the Four Generals of Takeda, and was also called the "Kenshin of Kai" for his outstanding tactical brilliance. His real name was Kasuga Toratsuna, and he was also known by the title Kōsaka Danjō. Even after Takeda Shingen's death, he continued to support his successor Katsuyori, but died of illness three years after the Battle of Nagashino, in Tenshō 6 (1578).

Masanobu was born the son of a local landowner in Kai Province (present-day Yamanashi Prefecture) and entered Takeda Shingen's service from a young age. Starting as Shingen's page, he grew into a senior retainer responsible for the northern Shinano region of the Takeda forces. He engaged fiercely in the Battles of Kawanakajima (five engagements from 1553-1564) against the Echigo forces under Uesugi Kenshin, where his tactical brilliance was fully displayed. Particularly in the Fourth Battle of Kawanakajima (1561), while the Takeda forces executed the woodpecker strategy, Masanobu played a key role as a member of the main force that met the Uesugi auxiliary unit withdrawing from Mijozan at Kawanakajima.

One of Masanobu's most important roles was to continually confront Uesugi Kenshin as lord of Kaizu Castle (present-day Matsushiro, Nagano Prefecture) in northern Shinano. Kaizu Castle was the cornerstone of Takeda control of northern Shinano, and Masanobu defended it for over twenty years. Its defenses were so formidable that even Uesugi Kenshin could not easily capture it. Receiving deep trust from Shingen, Masanobu is also said to be one of the authors (or original writers) of the "Kōyō Gunkan," playing an important role in transmitting Takeda military law and tactics to later generations.

In the realm of Japanese swords, Masanobu, as one of Shingen's closest attendants, witnessed the presentation and exchange of many famous blades. As lord of Kaizu Castle governing northern Shinano, he was deeply involved with the swordsmithing traditions of the region centered on the Zenkōji Plain, and is said to have patronized local smiths. The "Kōyō Gunkan" also contains many descriptions related to swords, preserving records of the Takeda way of using swords on the battlefield and famous blades. The sword Masanobu favored was passed down as a treasure of Kaizu Castle and became an important cultural property of the Matsushiro Domain (Sanada clan). Among the Four Generals of Takeda, Masanobu's life—known for his wisdom and martial-cultural excellence—embodies the ideal image of a Sengoku warrior.