Monarchy Versus Yangban

King Munjong's death in 1452 brought an 11-year-old Crown Prince to the throne. State affairs were left in the hands of state councilors, and monarchical power declined. In 1455, the unscrupulous Prince Suyang daegun, uncle of the child-king Danjong, usurped the throne by murder and regicide after quelling the opposition; he also ruthlessly suppressed attempts to restore Danjong as king.

King Sejo (r.1455-1468), as Prince Suyang daegun is officially known, closed the Hall of Worthies, abolished some posts in the Censorate Offices, and crippled the Office of Royal Lecturers (Gyeong-yeon), all measures designed to loosen the ideological restraints on the monarchy. The Office of Study Promotion was instituted, ostensibly as a means of promoting Confucianism. In fact, it was used merely as a royal library rather than as an organization designed to promote and propagate Confucian ideals. Further, he initiated the practice of giving private audiences to individual officials, flouting the regulation which made the presence of historians and censorate officials mandatory at royal audiences.

An attempt to raise the status of the monarchy was, however, justified, as the Korean monarch had formerly been vulnerable to inordinate yangban pressure. Yang Seongji, a talented scholar under King Sejong, advocated the monarch's cause in his memorials. Yang stressed Korea's unique position, asserting the need to preserve indigenous traditions. Dangun, according to him, was the "Son-of-Heaven Ruler." He formulated the proposition that Joseon, like China, was a nation upon which the "Mandate of Heaven" was bestowed. This argument strengthened King Sejo's hand via-vis the bureaucracy.

None of the Joseon kings had been strong enough to defy the yangban officials by praying in person at the Temple of Heaven, where the Son of Heaven alone was qualified to converse with the Heavenly God. King Sejo, however, in his sacrificial ode to the Heavenly God at the Temple of Heaven, used the phrase "the founder of the dynasty, the imperial great-great grandfather, Taejo (founder king Yi Seonggye)."

King Sejo ordered the compilation of a detailed map of Korea to provide further control of outlying areas. Census-taking of all soldiers and reserves in the various districts was enforced, and the Civil Register Act required all citizens to carry identification tags. He installed large military garrisons in each province and ordered every town to produce arms.

By arranging generous land grants and medicine, King Sejo showed his concern for the welfare of the army. He also ordered the migration of people to the sparsely populated northern border areas.

The monarch acted decisively in matters relating to the recruitment of new officials, increasing the number of military graduates to further strengthen the monarchial power. He also gave the title of "Meritorious Subject" to various officials on three different occasions to widen the base of loyal support. With the increase of inheritable land grants to meritorious subjects, however, land available for the newly appointed officials decreased. To solve this problem and to limit the economic power of the officials and yangban, King Sejo instituted the official land system, which allowed land grants in terms of rent for office tenure only. Thus, the status land system by which the yangban enjoyed life-time tenure was discontinued, and those parties who refused to compromise lost their land holdings altogether. This limitation of land grants to incumbent officials meant that the old landed yangban class changed to either an employed bureaucracy with land or landless yangban with prestige only.

King Sejo offered interim civil and military service examinations more often, in addition to the time-honored examinations given every three years. Since the number of successful candidates in the interim examinations exceeded those from fixed examinations by a ratio of two to one, this virtually brought the civil examination system under the monarch's sway.

To divert the attention of the Neo-Confucian scholars, King Sejo defied Confucian orthodoxy by supporting Taoism and Buddhism. An Office for Publication of Buddhist Scriptures was established, where the compilation of Buddhist literature and Korean translation of such literature became active. Fifty copies of the bulky Tripitaka Koreana were printed for distribution. To equip the often Sinocentric scholars with a comprehensive history of their own country, the compilation of Dongguk-tonggam (Comprehensive Mirror of the Eastern Kingdom) began in 1458 and was completed after the king's death.

During this time, the compilation of the Gyeongguk daejeon (Grand Code for State Administration) was initiated. The Gyeongguk daejeon became the cornerstone of the dynastic administration and provided the monarchial system with a sort of constitutional law in a written form.