Date Masamune (伊達政宗, Date Masamune) was born on September 5, 1567, the eldest son of Date Terumune (伊達輝宗, Date Terumune), daimyo (大名, daimyō) of a powerful clan in the Tohoku (東北, Tōhoku) region of northern Honshu. As a young child he contracted smallpox, which blinded and disfigured his right eye. The eye was eventually removed; accounts disagree on whether Masamune plucked it out himself after a rival taunted him, or whether his trusted retainer Katakura Kojuro (片倉景綱, Katakura Kojūrō) performed the removal at Masamune's request. Either way, the empty socket became his most famous feature — the physical mark of a man who made suffering into legend.

He became daimyo at approximately age 17 when his father was killed — kidnapped during a surrender negotiation by the neighboring lord Nihonmatsu Yoshitsugu and then shot in the chaotic rescue attempt. Masamune spent the next four years at war across Tohoku, systematically defeating every rival clan in the region. The decisive victory came at the Battle of Suriagehara (摺上原の戦い, Suriagehara no Tatakai) in 1589, where he defeated the Ashina clan and reportedly counted approximately 2,300 enemy heads. By his early twenties, Masamune controlled most of northern Japan. The age of the Sengoku (戦国, Sengoku) warlord was still burning, and he was its northernmost flame.

Then Toyotomi Hideyoshi (豊臣秀吉, Toyotomi Hideyoshi) brought his national unification campaign north. Hideyoshi summoned all daimyo to attend the Siege of Odawara in 1590. Masamune delayed — dangerously so, continuing his own campaigns when the summons required attendance. When he finally appeared before Hideyoshi, the story passed down is that he wore white robes: funeral dress, a signal that he expected to be executed for his tardiness and defiance. Hideyoshi reportedly laughed and spared him, but the punishment was severe nonetheless. Masamune's vast northern holdings were greatly reduced, and he was relocated.

He made the best of it. He chose a site in what is now Miyagi (宮城, Miyagi) Prefecture, surveyed it himself, and began building the castle town of Sendai (仙台, Sendai) in 1601. By 1604, it housed 52,000 people — vassals, merchants, craftsmen, and their families, all transplanted at the daimyo's command. The city Masamune built from a name and a hillside now holds more than one million people.

In 1613, Masamune financed one of the most ambitious diplomatic missions in Japanese history. His retainer Hasekura Tsunenaga (支倉常長, Hasekura Tsunenaga), born in 1571, led a 180-member expedition across the Pacific — through New Spain (Mexico), to Spain, and then to Rome. The ship, the San Juan Bautista, was built at Masamune's direction using European construction techniques. Hasekura received honorary Roman citizenship and noble rank from Pope Paul V in 1615. But by the time the mission returned to Japan in 1620, Tokugawa Ieyasu had issued anti-Christian edicts, the political landscape had shifted, and the mission had achieved nothing of substance. Hasekura died in 1622, reportedly a secret Christian. The descendants of his mission's Japanese Catholics still live in Coria del Río, Spain today, many carrying the surname "Japón."

Masamune spent his final decades as a peacetime lord under the Tokugawa order — constrained, watched closely, never fully trusted, never able to campaign again. He was a war-forged commander with no wars to fight. He reportedly penned some 1,300 of the 4,500 letters attributed to him personally, a prolific correspondent who was said to take pride in his calligraphy. He outlived Toyotomi Hideyoshi (died 1598), outlived Tokugawa Ieyasu (died 1616), outlived every rival and contemporary of the Sengoku generation.

On June 27, 1636, Date Masamune died at the Date domain residence in Edo, of esophageal cancer complicated by peritonitis. He was 68 years old. Shogun Tokugawa Iemitsu (徳川家光, Tokugawa Iemitsu), alarmed by the daimyo's condition when he arrived in Edo for his annual sankin-kotai (参勤交代, sankin-kōtai) attendance, had summoned dozens of doctors. None could save him. Iemitsu visited the dying man personally — an act of extraordinary respect that the shogunate rarely extended to vassals, however celebrated. Masamune was then carried back to Sendai in the same formal procession that would have accompanied a living lord. His mausoleum, the Zuihoden (瑞鳳殿, Zuihōden), was built the following year and still stands in Sendai on a wooded hill, where archaeologists who opened his tomb in 1974 confirmed his skeleton — including the distinctive empty right eye socket.

The nickname dokuganryu (独眼竜, dokuganryū) — "One-Eyed Dragon" — was not used during his lifetime. It first appeared in a poem by the Confucian scholar Rai San'yo (頼山陽, Rai San'yō) in 1830, nearly 200 years after Masamune's death. In Japanese tradition, a ryū (竜, ryū, dragon) is a creature of immense and unpredictable power; "dokugan" (独眼, dokugan) means "single eye." The legend, as so often happens with legends, grew larger after the man was gone.