He had been fighting wars since he was fourteen. By the time the Sekigahara Campaign began in the summer of 1600, Date Masamune had already conquered, lost, reclaimed, and been forced to surrender most of the Tōhoku region in a career of almost continuous military action. He was 32 years old, one-eyed since childhood — smallpox had taken the right eye — and famous enough that his enemies had stories about him before they met him. Whether he had removed the ruined eye himself, or had a trusted retainer do it to prevent an enemy from grabbing it in close combat, was already a matter of legend rather than fact.

In the summer of 1600, Masamune's calculation was simple and very dangerous. Tokugawa Ieyasu was marching east against the Uesugi clan, who had declared for the Western Army coalition forming under Ishida Mitsunari. Ieyasu needed northern allies, or at least northern lords who were not actively stabbing him in the back. Masamune offered nominal loyalty — and simultaneously mobilized 20,000 men against Shiroishi Castle, a fortress held by the Amakasu clan as Uesugi retainers, sitting directly south of his domain.

He took the castle with fire attacks. He installed his chief advisor, Katakura Kojurō Kagetsuna, as its commander. The Katakura clan would hold Shiroishi for eleven generations — more than two centuries — as senior retainers of the Sendai domain. Masamune had added a significant fortress to his holdings while nominally acting in support of the Tokugawa cause. The logic was elegant: if Ieyasu won, Masamune had fought for him. If Mitsunari won — well, the letters to the Western Army had been carefully worded too.

Ieyasu survived, won at Sekigahara, and looked north. He had, before departing, issued Masamune an "approval for one million koku" — a promise that he could claim whatever he could take. After the battle, Ieyasu honored the promise and clarified its terms. Masamune's confirmed domain came to roughly 620,000 koku. The million was a ceiling, not a floor. He would found the city of Sendai, govern it well, and spend the next three decades being the most interesting man in a room that no longer had room for interesting men.

The epithet that defines him — 独眼竜 (dokuganryū), the One-Eyed Dragon — is itself a fascinating piece of posthumous myth-making. The combination of 独 (doku — single/alone), 眼 (gan — eye), and 竜 (ryū — dragon) suggests a warrior who is ferocious, elemental, and marked by battle. In Chinese and Japanese tradition, the dragon represents supreme power. The one-eyed qualifier adds something else: damaged but undimmed. The name seems to have been waiting for him. Except it wasn't coined until 1830, by the Confucian scholar Rai Sanyō, in a poem written to commemorate the great northern lord. Two centuries after Masamune's death in 1636, a poet named him. The legend had been traveling on its own momentum the whole time.