Somewhere in Edo Castle, two days into his war council, Tokugawa Ieyasu was processing the consequences of a letter. The armies were forming. The western coalition was mobilizing. And the document that had set all of it in motion was a piece of correspondence from a man who had decided that courtesy was beside the point.

Naoe Kanetsugu was karō (karō) — chief retainer — to Uesugi Kagekatsu, lords of a great domain in what is now Fukushima Prefecture. When Ieyasu sent a Buddhist monk, Seishō Jōtai, to demand that Kagekatsu explain his allegedly treasonous fortification of his domain, the monk returned with a letter. The Naoejō did not explain. It did not apologize. It listed Ieyasu's own violations of the orders left by the late Toyotomi Hideyoshi, the man both sides claimed to serve.

The letter was sent through the same monk who had delivered Ieyasu's challenge — a chain of courtly protocol wrapping around one of history's most explosive correspondences. Ieyasu declared the Uesugi in rebellion and led 56,000 troops northeast. The war had begun.

Kanetsugu is not easily categorized. He was a brilliant administrator, a fierce loyalist, and a man who understood that words could be weapons. He also wore into battle a kabuto (kabuto) — a helmet — bearing the single kanji 愛 (ai), which he interpreted not as romantic love but as Aizen Myōō, the Buddhist deity of fierce devotion. The man who wrote the most insulting letter in samurai political history went to war under a symbol of love.

Whether the Naoejō was a genuine taunt, a calculated trap designed to draw Ieyasu east while Ishida Mitsunari mobilized in the west, or simply a point of honor — Kagekatsu had done nothing wrong, and his retainer refused to say otherwise — historians cannot agree. What is not in dispute is the effect: Ieyasu marched, Mitsunari moved, and within ten weeks Japan had fought the battle that would determine the next two and a half centuries.

There is one further complication. The original letter does not exist. The copies that circulated from 1654 onward — more than fifty years after the letter was supposedly written — contain grammatical constructions and honorific usages that modern scholars say were not current in 1600. It is entirely possible that Japan's most famous political provocation is a later literary invention, composed by admirers who wanted Kanetsugu's defiance to be as eloquent on paper as it apparently was in deed.

When Kagekatsu heard of Kanetsugu's death in 1620, the famously impassive daimyo wept. Whatever the letter was or was not, the man behind it was mourned with tears.