In November 1945, the United States Army Military Government in Korea (1945-48) began the task of organizing Korean military and police forces. In December a school for training military officers was established the South Korean National Constabulary was organized in January 1946. The United States originally had planned to assist South Korea in developing only those police and military organizations necessary to maintain law and order during the period Korea was to be under the five-year Soviet-American trusteeship. By 1948, however, it was apparent that South Korea would need to expand the National Constabulary into a larger and more conventionally organized army to adequately defend itself from a possible invasion by North Korea. For this reason, the United States provided funds and training to expand the eight provincial units and one capital city unit of the National Constabulary from regiments to brigades (see South Korea under United States Occupation, 1945-48 , ch. 1).

In November 1948, the Republic of Korea National Assembly passed the Armed Forces Organization Act. Under the provisions of this act, the National Constabulary was reorganized into an army comprising seven divisions. In June 1949, when the last United States Army units deployed in Korea as part of the post-World War II occupation forces withdrew, leaving behind a 500-person military advisory group, the leaders of the South Korean army controlled an organization that had been internally weakened by subversion and political factionalism and that lacked enough trained personnel and modern weapons to prepare adequately for war.

North Korea's effort to win control of the south using guerrilla warfare forced South Korea's military leaders to concentrate on counterinsurgency operations. In the fall of 1949, North Korean guerrilla units attempted to gain control of remote areas and small towns in the mountainous areas of eastern and southern South Korea. It was estimated that as many as 5,000 guerrillas trained in North Korea were infiltrated into these areas by the winter of 1949. Two South Korean army divisions and one army brigade were quickly deployed to conduct sweep and destroy missions to eliminate the guerrillas. Counterinsurgency operations were initiated in South Cholla Province in October 1949. In some areas, South Korean villages were evacuated both to protect civilians and to assist counterinsurgency units in locating guerrilla bases. By April 1950, less than 500 North Korean guerrillas remained in South Korea. Although the counterinsurgency program succeeded in ending the threat posed by the guerrillas, it had a deleterious effect on the army, necessitating reorganization and retraining for conventional war preparedness.

Data as of June 1990