Sejong (1397-1450) was a Korean king and inventor of the Korean alphabet. His long reign, 1418-1450, is generally acknowledged to have been the most brilliant period of the Yi dynasty.

Sejong, formally named Yi To, and known as Prince Ch'ungnyong before taking the throne, was born on May 7, 1397. Since his eldest brother, Prince Yangnyong, had been designated crown prince in 1404, Sejong in early life did not anticipate the throne. But Yangnyong's erratic behavior led to his deposition and Sejong's elevation in July 1418. (An unofficial tradition holds that Prince Yangnyong feigned instability so that Sejong might rule.) Sejong became king, following his father's abdication, on Sept. 7, 1418.

During his reign Sejong introduced improved administrative procedures, new census laws, penal reforms, and some civil rights for slaves and outcast groups. He maintained proper relations with China and a firm stance with Japan. His devastating attack on the island of Tsushima in...