On June 8, 1600, Tokugawa Ieyasu (徳川家康) sent word to every allied daimyo (大名) in Japan: march with him against Uesugi Kagekatsu (上杉景勝), or be counted among his enemies. Ieyasu was 57 years old, the most powerful man in Japan since Toyotomi Hideyoshi's (豊臣秀吉) death two years earlier — and he had just been called a liar and a hypocrite in writing by a vassal of the lord he was preparing to destroy. The call to arms on June 8 was the only response left to him.

The sequence of events began in the months before. Uesugi Kagekatsu (上杉景勝), lord of Aizu (会津) domain with holdings of 1.2 million koku (石), had been making aggressive preparations — fortifying his castle, recruiting soldiers, accumulating weapons. To Ieyasu, this looked like preparation for open defiance of the Tokugawa-led council of regents governing Japan on behalf of Hideyoshi's young heir. In May 1600, Ieyasu sent a formal letter of inquiry demanding an explanation.

What came back changed the course of Japanese history. Naoe Kanetsugu (直江兼続), Kagekatsu's chief advisor and approximately 40 years old, composed the reply that would become known as the Naoe-jō (直江状) — the Naoe Letter. According to traditional accounts, the document did not apologize. It attacked. Kanetsugu listed Ieyasu's own violations of Hideyoshi's dying instructions: the alliances Ieyasu had made without authorization, the marriages he had arranged in defiance of Hideyoshi's rules, the political maneuvering that had accumulated power in Ieyasu's hands while paying lip service to the Toyotomi succession. In the most celebrated reading of the letter, Kanetsugu was essentially writing: you are the one who has been breaking the rules. Come at us if you dare.

Historians note that the surviving text of the Naoe-jō may be an embellished version of the original — the exact wording cannot be fully verified, as only copies survive. But the letter's effect is beyond doubt. Ieyasu could not absorb such a challenge to his authority without responding. Every daimyo in Japan was watching. If Kagekatsu could mock him in writing and face no consequences, Ieyasu's position would be fatally undermined.

On June 8, 1600, Ieyasu issued his formal call to arms. His allied daimyo were summoned to a meeting at Osaka on July 12, where he gathered support from major eastern lords including Hosokawa Tadaoki (細川忠興), Date Masamune (伊達政宗), and Mogami Yoshiaki (最上義光). By mid-July, Ieyasu was marching east with an army of approximately 56,000 soldiers — the largest military force assembled in Japan in decades — heading toward Aizu through the summer heat.

He never reached it. While Ieyasu moved east, Ishida Mitsunari (石田三成) — a former Toyotomi administrator who despised Ieyasu — raised what would become the Western Army in Ieyasu's absence. The military buildup in Aizu, some historians have speculated, may have been as much bait as defiance: draw Ieyasu east, then strike behind him. Whether or not Kagekatsu and Mitsunari had coordinated the timing, the effect was the same. Ieyasu turned his army around and marched west.

On October 21, 1600, the Eastern Army and the Western Army met in the mountain fog at Sekigahara (関ヶ原). The battle lasted a single day. Ieyasu's forces won decisively. The Western Army was destroyed. Kagekatsu's domain was reduced from 1.2 million koku to 300,000 koku as punishment for his defiance. Mitsunari was captured and executed. In 1603, Ieyasu received from the emperor the title of Sei-i Taishōgun (征夷大将軍), formalizing what the battle had already decided. The Tokugawa dynasty he founded ruled Japan for 265 years.

The chain of causation runs straight back to June 8, 1600 — and to the letter that forced the day to happen. Naoe Kanetsugu wrote a document intended to defend his lord's honor. In doing so, he handed Ieyasu the pretext he needed, pulled every daimyo in Japan into a choosing of sides, and set the dominoes falling toward Sekigahara. The retainer who refused to bow inadvertently helped build the dynasty that would rule Japan until 1868.