It is necessary to examine why Joseon executed Nangbarahan(낭발아한) to understand the cause of Joseon’s conquest of Moryeonwi in King Sejo’s reign. Joseon tried to eliminate strong Jurchen chiefs when they did not remain under its control. By doing so, Joseon sought to eliminate those who could serve as pivotal figures in Jurchen and to expand Joseon’s influence.
Joseon’s conquest of Moryeonwi was not in revenge for invasion from Yain. There already was a consensus across Joseon that it should now take on a foreign conquest. A conquest means to exert extreme force on external powers. This requires not only a preparatory period but also careful consideration of aftermaths of the conquest and financial expenses. Particularly, its failure holds the risk of a heavy blow to the authority of Joseon and its king. In light of this, invasions from Abiguh (아비거) and others are seen to have been used as a justification for the conquest, rather than its direct cause.
Sejo launched a conquest of Moryeonwi earlier than originally scheduled, and ordered that the envoy to Ming be brought to Pyeongyang, where his arrival was previously forbidden. Sejo also went on a patrol of northern regions. During his patrol. Sejo intended to meet the Ming envoy in Pyeongyang and receive his reports on the success of the conquest of Moryeonwi. Sejo wanted these three rituals to proceed in Pyeongyang simultaneously so as to strengthen his royal authority further.
Joseon kept in touch with Yain in the following years. At the same time, its conquest had been more frequently discussed than before. Joseon was not damaged by neighboring forces at the time. It rather tried to control them with its overwhelming military force. Joseon’s conquest of Jurchen was not in response to invasions from Yain. Its purpose was to make Yain surrender to Joseon by exerting military force. Using the outcome of the conquest in his ruling of Joseon, King Sejo sought to expand his political influence and strengthen the royal authority.
Since King Taejong came to power, Joseon emphasized obedience and subservience to Ming. Joseon in the 15th century, however, often showed an ambivalent attitude toward Ming. A representative example of this is its relation with Jurchen, specifically its conquest. Obedience held important value for Joseon, but it could be taken advantage of at any time to consolidate the royal authority.