The enemy was already inside the province.

Twenty-eight days before his birth, an Imagawa (今川, Imagawa) army roughly fifteen thousand strong marched into Kai Province (甲斐国, Kai no Kuni) under the command of Fukushima Masashige (福島正成, Fukushima Masashige), acting under orders from Imagawa Ujichika (今川氏親, Imagawa Ujichika) of Suruga. The Takeda estate was no longer safe. Lady Oi (大井の方, Oi no kata) — daughter of a powerful kokujin (国人, kokujin) clan from Nishigoori, and heavily pregnant with her first son — fled to Sekisui Temple (積翠寺, Sekisui-ji) in the mountains above Kofu (甲府, Kofu). She gave birth there on December 1, 1521.

Her husband, Takeda Nobutora (武田信虎, Takeda Nobutora), was at the same moment commanding his forces at the battle of Iidagawara (飯田河原の戦い, Iidagawara no Tatakai) — a desperate engagement meant to stop the Imagawa advance from becoming a rout. According to the chronicle Kohakusaiki (甲斐国志, Kohakusaiki), news of the healthy son reached the Takeda lines during the fighting. Morale surged. The Takeda drove the Imagawa back.

The boy's childhood name was Katsuchiyo (勝千代, Katsuchiyo) — 'victory' and 'a thousand years.' Later he would take the adult name Harunobu (晴信, Harunobu) at his genpuku (元服, genpuku), the coming-of-age ceremony, borrowing a character from the reigning Ashikaga (足利, Ashikaga) shogun as a signal of the Takeda's political ambitions. The name history remembers — Shingen (信玄, Shingen) — was the Buddhist name he adopted later in life.

Nobutora did not see greatness in his first son. He favored his second son, Nobushige (信繁, Nobushige), and by the mid-1530s was reportedly making arrangements to disinherit Harunobu entirely. The slight may have sharpened what was already there: at the battle of Un no Kuchi (海ノ口の戦い, Un no Kuchi no Tatakai) in 1536, the fifteen-year-old Harunobu led a company of mounted troops that helped turn the battle. His father noted the performance without changing his mind about the succession.

The coup came in 1541. Harunobu, then nineteen, had spent years cultivating the clan's disaffected retainers, building a faction that owed its loyalty to him rather than his father. The moment his coalition was solid enough, he moved — not with violence, but with something more surgical. He approached the Imagawa, the same clan whose invasion had driven his mother to that temple twenty years before. The Imagawa agreed to receive Nobutora as a permanent guest. One day, Nobutora found himself on a boat crossing into Imagawa territory. He never returned to Kai.

The bloodless precision of the coup announced what Shingen would become: a man for whom war and diplomacy were a single instrument. He spent the next three decades building the Takeda into the most formidable military machine in eastern Japan.

His armies moved under the banner of 風林火山 (Fūrinkazan) — four principles taken from Sun Tzu's (孫子, Sunshi) Art of War: swift as the wind (疾如風, hayaki koto kaze no gotoshi), silent as the forest (徐如林, shizuka naru koto hayashi no gotoshi), fierce as fire (侵掠如火, okasazu toru koto hi no gotoshi), immovable as a mountain (不動如山, ugokazu koto yama no gotoshi). His cavalry wore lacquered red armor. The visual impact of a Takeda charge — hundreds of crimson horsemen, the banner rippling at their back — was designed to shatter opponents psychologically before a weapon was raised.

For twelve years (1553–1564), Shingen fought the series of battles on the plain of Kawanakajima (川中島の戦い, Kawanakajima no Tatakai) against his great rival Uesugi Kenshin (上杉謙信, Uesugi Kenshin) of Echigo (越後, Echigo). The two men were so evenly matched that the battles produced no decisive result — only a legacy so dramatic that later generations turned Kawanakajima into the defining image of Sengoku single combat.

The final campaign began in the autumn of 1572. Shingen marched west at the head of a Takeda army, announcing an intention to reach Kyoto. At the battle of Mikatagahara (三方ヶ原の戦い, Mikatagahara no Tatakai) on January 6, 1573, he destroyed a Tokugawa-Oda allied force, leaving Tokugawa Ieyasu (徳川家康, Tokugawa Ieyasu) to flee back to Hamamatsu Castle (浜松城, Hamamatsu-jo) in such haste that legend records him soiling himself in the saddle — a story Ieyasu was said to have commissioned as a painting to remind himself never again to be that reckless.

Shingen sent a letter to Oda Nobunaga (織田信長, Oda Nobunaga) after the battle, essentially telling him to yield. Nobunaga, who was at that moment fighting multiple enemies simultaneously, reportedly told his own council he did not know what he would do if Shingen reached Kyoto. No other daimyo in the era drew that admission from him.

Shingen never reached Kyoto. In the spring of 1573, while his army was besieging Noda Castle (野田城, Noda-jo) in Mikawa (三河, Mikawa), he fell ill. The cause is disputed — pneumonia, an old wound, or a sniper's bullet from the castle walls, depending on the source. He died on May 13, 1573, at fifty-one, somewhere on the road back to Kai.

His final instructions were characteristic: keep his death secret for three years, hold the clan together, wait. The generals obeyed for as long as they could — perhaps a few months. By 1575, at the battle of Nagashino (長篠の戦い, Nagashino no Tatakai), Oda Nobunaga's arquebusiers destroyed the famous Takeda cavalry and shattered the clan as a military force. In 1582, the Takeda were obliterated entirely.

But in December 1521, none of that had happened yet. A woman sheltered in a mountain temple. An army fought outside. A boy was born.