The child had a cold.

That is how it began — or rather, how it ended. Tokugawa Ietsugu (徳川家継, Tokugawa Ietsugu) had been installed as the seventh shōgun (将軍, shōgun) of the Tokugawa dynasty at the age of three, following the death of his father, Tokugawa Ienobu (徳川家宣, Tokugawa Ienobu). He never governed. He could not. Real power since his installation had been exercised by two holdovers from his father's court: the brilliant Confucian scholar-adviser Arai Hakuseki (新井白石, Arai Hakuseki) and the lord chamberlain Manabe Akifusa (間部詮房, Manabe Akifusa). They ran the bakufu (幕府, bakufu) — the shogunal government — in a child's name, drafting the edicts, receiving the daimyō (大名, daimyō), and managing a realm of twenty-five million people while their nominal master sat on a throne too large for him and played.

History's only record of Ietsugu's personality is that he was playful and mischievous. He was also, at the age of four, betrothed to a one-year-old imperial princess — Yoshiko-no-Miya (よし子内親王, Yoshiko-no-Miya), daughter of Emperor Reigen (霊元天皇, Reigen Tennō). Neither child could walk to the ceremony unaided. It was, like everything about Ietsugu's life, a performance staged entirely for the benefit of adults who needed him to represent things he could not yet comprehend.

Sometime in the late spring of 1716, he fell ill with a cold. By the standards of the Edo period, this was not initially alarming — children caught colds constantly. But a cold became a cough, a cough became a fever, and a fever in a child of six, without modern medicine, became what Japanese sources would later describe as 急性肺炎 (kyūsei haien) — acute pneumonia. On the 30th day of the 4th month of Shōtoku 6 (正徳6年4月30日), Ietsugu died in Edo Castle (江戸城, Edo-jō). He was six years old by Western reckoning. He was buried at Zōjōji Temple (増上寺, Zōjōji) in Edo, the traditional mausoleum of the Tokugawa shoguns, in a tomb that is the smallest in that long row of powerful men.

The crisis was immediate and existential. Ietsugu had no son, no male heir of any kind. His death extinguished the direct male line of Tokugawa Iemitsu (徳川家光, Tokugawa Iemitsu), the third shōgun — the man who had closed Japan's ports, expelled the Portuguese, and bound the country in the isolation policy of sakoku (鎖国, sakoku). For the first time since Ieyasu had founded the dynasty in 1603, the Tokugawa had no clear successor.

The rōjū (老中, rōjū) — the senior council of elders — convened that night with the heads of the Gosanke (御三家, Gosanke): the three great Tokugawa collateral houses of Owari (尾張, Owari), Kii (紀伊, Kii), and Mito (水戸, Mito). By rank, Owari was senior. By logic, Owari's candidate should have been chosen. But Ietsugu's mother, Gekkō-in (月光院, Gekkō-in), had spent the weeks before her son's death lobbying with quiet intensity for a different man: Tokugawa Yoshimune (徳川吉宗, Tokugawa Yoshimune), the thirty-three-year-old daimyō of the Kii domain.

Yoshimune was not the obvious choice. He was not the most senior. But he had a reputation that the Owari candidate lacked: he had run his own domain with iron frugality and genuine capability, cutting waste, reviving martial training, personally leading his retainers in flood-control labor. He dressed simply. He ate plainly. He was said to go hawk-hunting dressed as a commoner and accept rice lunches from peasants along the way — a deliberate theater of the old warrior spirit that stood in deliberate contrast to the perfumed luxury of the Genroku era (元禄時代, Genroku Jidai). A grieving mother's lobbying, combined with Yoshimune's reputation as the man most resembling Ieyasu in spirit, tipped the deliberation.

Yoshimune was selected as the eighth shōgun.

Within days he had dismissed both Arai Hakuseki and Manabe Akifusa, ending the era of governance-by-adviser and restoring direct shōgunal authority for the first time in a generation. He moved himself out of the shogun's lavish formal quarters and into a plain antechamber. Then he began the 享保の改革 (Kyōhō no Kaikaku) — the Kyōhō Reforms — the most sweeping reorganization of the Tokugawa system since the dynasty's founding.

The reforms ran for two decades: cutting shogunal expenditures, imposing the agemai (上米, agemai) rice levy on daimyō domains to refill the bankrupt treasury, reviving samurai martial training after the soft decades of the Genroku boom, introducing merit-based appointment alongside hereditary rank, and opening the shogunate to petitions from ordinary subjects via the meyasubako (目安箱, meyasubako), a suggestion box placed outside the castle gate. Yoshimune personally read the submissions.

He ruled until 1745 — twenty-nine years — and is consistently ranked by Japanese historians as one of the three greatest Tokugawa shoguns, alongside Ieyasu and Hidetada (秀忠, Hidetada). The Tokugawa dynasty survived another 152 years after his accession. Without the crisis of Ietsugu's death forcing a fresh outsider into power, historians have wondered whether the dynasty's slow institutional rot would have continued unchecked — luxury compounding luxury, adviser compounding adviser, until there was nothing left but ceremony.

Instead, a six-year-old caught a cold. His dynasty got 150 more years.