At the time that the Yi Dynasty gained power in Chosun (Korea), the organization of the military still included both Royal and Nobility privately controlled formations. The Yi began a series of reorganizations and by 1460 a new national army structure was in place. This structure and the military obligations placed on the people was retained by the Yi rulers. The strict caste system which existed throughout the country was reflected in the military as well. The regularly trained core troops came from a social class, yangban, who were required based on birth to be soldiers or civil government officials. The junior officers were derived from a higher class. The senior commanders and Generals were all drawn from the upper nobility. One advantage of a military profession being determined by birth was that military training and the attendance at military schools was provided to all designated youth. As a result the level of military training for Korean soldiers at all levels was very high. One disadvantage of the it being a caste system it was almost impossible for a soldier to be promoted above his station based on merit or valor.

In addition to the soldier class, the lower common classes were required to provide service in times of national emergency, such as invasion. Also they were required to spend a set number of days per year either in public works service or conducting military training. Likewise middle class artisans and craftmen were required to serve in emergencies and to pay part of their taxes by making military items such as spears, bows, arrows and armor. The taxes of textile artisans could be paid in part by providing cloth to the government which could be used to pay tribute to China or to provide uniforms for the government's troops.

The highest troop classification were the Bodyguards who were assigned to protect the royal family and other government officials. They were mainly trained as policemen rather than soldiers and were regarded as highly loyal but poor combat troops. The next level of troops were actually regarded as the best warriors and drawn from the Yangban middle class. These troops often referred to as 'armored soldiers' were brigaded in elite units and assigned either to the capital or to the frontier. They would often fight mounted (depicted charging in a wedge in many paintings) and would have the best armor and equipment. The third level of troops were the common soldiers. By 1469, these were full-time volunteers from the lower social classes. They were often deployed as garrison troops in the less threatened regions or to supplement the garrison of a vital region. The fourth level were the conscripts who were often impressed into service only under emergency conditions. They were trained mainly as spearmen though some operated as archers. A final group which has to be mentioned are the various rebel bands which were raised. Unlike during the earlier dynasties, the number of peasant revolts during the YI dynasty were limited. The greatest rise of such groups occurred during the Japanese invasions of the 1590s. Though many of the rebels were farmers, most groups which fought the Japanese seem to have a core of soldier refugees in them. One group was comprised entirely of Buddhist monks operated closely with the Ming forces sent to aid the Koreans.

The national regular army was divided into five garrisons that per assigned to one of the five military districts. A Division was assigned to one of five regions (Center, North, West, East and South) of the district. Each Division was subdivided into five operational formations. They had five Pu (Brigades), each of four T'ong (Regiments), with Yo (Battalions), Tae (Companies) and O (Platoons).

Each administrative province maintained a military headquarters which housed a varying number of army and naval commanders. The number of these commanders varied with their particular strategic situation. A border province may have only one naval commander but several page 2 army commanders. On the other hand, a coastal province near Japanese islands might have several army and naval commanders in order to respond more effectively to raids by Japanese pirates. Operationally, the district's Senior General would dispatch the number of troops needed to defeat the enemy under a subordinate general. He would retain the majority of his command as a reserve. Part of the reserve could be dispatched later to reinforce a deployed force or to respond to a new threat. This deployment practice is why a district headquarters would support multiple commanders. Also commanders were used to control the local garrisons of strategic towns or fortified passage points.

While the Korean military was well organized and trained on paper, in actuality by 1590 it's effectiveness had deteriorated. During the long period of relative peace, the required period of annual military training had been used to conduct public works construction. In addition many middle and lower class men were paying exemption from service taxes in textiles which further reduced the number of trained common soldiers.

When the Japanese invasion occurred in 1592 most of Chosun's best troops were deployed in the north. These were deployed there to respond to the numerous raids by Manchurian tribes. Once the invasion occurred, they could not be redeployed quick enough to save many of the southern towns. Eventually, these forces operated with the Ming troops sent to aid the Koreans.

Chosun: The Isolationists of East Asia

Introduction
Military Organization and Notes
The Neighbors of Chosun Korea
Uniforms of the Yi Dynasty Era
Chronology of Military Actions in Chosun/Korea 1340-1700