From MangaHelpers:

This is a story set in the dangerous Sengoku period, featuring the famous historical figure of Takeda Harunobu, later called Shingen. When he was fifteen, he accompanied his father to the Battle of Un no Kuchi, where he won his first victory as his father retreated. After that, his relationship with his father, Takeda Nobutora, changed. He heard rumors that his father wanted to give his inheritance to Harunobu's younger brother. Soon, Harunobu became certain that he must do something about his father...