Sejo, formally named Yi Yu, and known as Prince Suyang before taking the throne, was born November 7, 1417, the second son of the great king Sejong. Among eight royal heirs Sejo was perhaps the most capable, but since the designation as crown prince had gone to his elder brother (Yi Hyang, who reigned as Munjong, 1450-1452), Sejo was from the beginning cut off from the succession.
Throughout Sejong's reign the royal brothers worked well together, directed by an even-handed father who kept them busy. Sejo was well informed on military affairs, having observed frontier operations against the Jürchen and participated in the development of munitions and ordnance during the early 14406. He made a major contribution as director of his father's land-survey commission; the formulas developed by his body for measuring crop yields and assessing taxes became a fundamental part of Yi-dynasty fiscal structure. Sejo as a prince also wrote in the vernacular an account of the Buddha's life, which provided the inspiration for his father's Buddhist poetry. Sejo's brother Munjong succeeded Sejong in 1450 and was competent enough as king; but his health was poor, and he quickly became a target for bureaucrats and officials. Sejo and his younger brother, Prince Anp'yong (1418 - 1453), helped their weaker brother by seeing that the throne's interests were asserted. But when on the King's premature death, in 1452, Munjong's 10-year-old son, Tanjong (Yi Hongwi, also known as Prince Nosan), succeeded, a rift grew between the two brothers over the exercise of power during Tanjong's minority. Finally, in November 1453, Sejo, charging Prince Anp'yong and his followers with plotting to overthrow the young king, banished his brother to an island, where he was forced to commit suicide, and murdered the principal men of his faction. With Sejo now in complete control, Tanjong grew increasingly edgy. Convinced that Sejo's coups were not yet over, he abdicated on July 25, 1455. Sejo took the throne the same day, while Tanjong moved to a lonely exile in remote Kangwon Province. Sejo justified his usurpation on the grounds that unless a strong king sat on the throne the royal power would steadily be eroded. But many men of his day felt he had gone too far, and inevitably a movement grew to restore Tanjong. A group of loyalists planned a coup for July 1456 but were betrayed by an informer. Sejo personally carried out their interrogation, subjecting the six plotters to unspeakable torture and mutilation. The historical accounts of this confrontation show the six men composing defiant poetry and lecturing Sejo to silence before finally breathing their last. Soon afterward Sejo ordered Tanjong's suicide. These murders echoed through the centuries that followed, with the overwhelming opinion falling on the side of the boy king and the "Six Dead Ministers, " as they came to be called. (Six sympathetic officials who went into lifetime retirement to protest Sejo's action are called the "Six Live Ministers. ") Tanjong and all loyalists were posthumously rehabilitated during the 17th century, but long before that the reverberations of the affair had precipitated factional struggles and purges (notably in 1498, when defense of the loyalists was adjudged lèse-majesté). From 1456 on, Sejo's power was not again questioned. Sejo abdicated in favor of his son Yejong on September 22, 1468, and died the next day of an incurable disease.
He had his way in virtually everything, and it can be said in his favor that, once established, he was a remarkably effective king. Among his achievements were lavish support of Buddhist writings and their publication, effective frontier defense, suppression of a major rebellion, and institution of the "secret censor" system, by which royal spies circulated covertly through the provinces ferreting out and summarily punishing corruption. In time these posts became themselves major focuses of graft, but the original idea of incorruptible censors had a long life in popular fiction.
