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Sejong the Great

In August 1418, following Taejong's abdication two months earlier, Sejong the Great ascended the throne. In May 1419, King Sejong, under the advice and guidance of his father Taejong, embarked upon the Gihae Eastern Expedition to remove the nuisance of waegu (coastal pirates) who had been operating out of Tsushima Island.

In September 1419, the daimyō of Tsushima, Sadamori, capitulated to the Joseon court. In 1443, The Treaty of Gyehae was signed in which the daimyō of Tsushima was granted rights to conduct trade with Korea in fifty ships per year in exchange for sending tribute to Korea and aiding to stop any Waegu coastal pirate raids on Korean ports.

On the northern border, Sejong established four forts and six posts to safeguard his people from the Jurchens, who later became the Manchus, living in Manchuria. In 1433, Sejong sent Kim Jong-seo, a government official, north to fend off the Jurchens. Kim's military campaign captured several castles, pushed north, and expanded Korean territory, roughly the present-day border between North Korea and China.

During the rule of Sejong, Korea saw advances in natural science, agriculture, literature, traditional Chinese medicine, and engineering. Because of such success, Sejong was given the title "Sejong the Great". The most remembered contribution of King Sejong is the creation of Hangul, the Korean alphabet, in 1443; rejected in its time by the scholarly elite, everyday use of Hanja in writing eventually was surpassed by Hangul in the later half of the 20th century.