Within days of North Korea's June 25, 1950, invasion across the 38th parallel into South Korea, President Harry Truman and his military advisors made the decision to mobilize part of the Reserve components of the Army National Guard. On June 30, 1950, the Selective Service Extension Act continued the military draft in effect since 1948 and authorized the president to order Reserve component units into federal service for a period not to exceed 21 months (later increased to 24 months).

In the days immediately following the North Korean invasion, mobilization planners envisioned activating only a limited number of specialized Reserve units. However, after a string of military disasters on the Korean Peninsula forced U.S. and Republic of Korea (ROK) troops steadily back toward their final defensive perimeter around the port city of Pusan, it became obvious that more Reserve and National Guard troops would be needed. Accordingly, the first Army National Guard units alerted for federalization in July 1950 included the 40th (California) and 45th (Oklahoma) Infantry Divisions, both of which reported for active duty on Aug. 14, 1950. Two weeks later Pennsylvania's 28th Infantry Division and the 43d from the New England states, were also federalized. These two divisions were sent to Germany to shore up U.S. strategic defenses there; the 40th and 45th would later see combat in Korea.

The mobilization of the Army National Guard for Korean War service occurred in 19 separate increments, with units reporting for active duty between Aug. 14, 1950, and Feb. 15, 1952. It included eight infantry divisions, three regimental combat teams, and 714 company-sized units.

The 138,600 personnel federalized with their units represented about one third of the Army National Guard's total strength. Many guardsmen went to Korea not with their units, but as individual replacements for units already in theater.

The first Army National Guard units reached South Korea in late December 1950 as the U.S.-led United Nations Command was recovering from the shock of the Chinese communists' entry into the war two months before. The forces of the U.S. and its U.N. allies, which had driven through North Korea almost to the Manchurian border, were forced to retreat after one of the worst defeats an American army has ever suffered. Later, the regrouped Eighth U.S. Army mounted limited offensives against the Chinese and North Koreans, and as spring approached was joined by increasing numbers of Army National Guard units, including many engineer, field artillery and transportation units.

Enemy forces launched another major offensive during spring 1951, and recaptured Seoul for the third time. U.S. artillery, often firing at point-blank range, was a major weapon against the terrifying "human wave," attacks which had come to characterize the fighting in Korea. Three Army National Guard artillery battalions, the 196th Field Artillery (Tennessee), 937th Field Artillery (Arkansas), and 300th Armored Field Artillery (Wyoming) won Presidential Unit Citations for supporting U.S. Army and Marine Corps divisions.

As their company- and battalion-sized counterparts arrived in South Korea to join the fighting, the Army National Guard's 40th and 45th Infantry Divisions were training in Japan. While there had been talk of leaving the two divisions in Japan, in November 1951 the decision was made for a "swap in place" between the two Guard divisions and the 1st Cavalry and 24th Infantry Division. In December 1951, the 45th Division arrived in South Korea to take over the positions, equipment and mission of the 1st Cavalry Division; in January, the process was repeated by the 40th and the 24th Infantry Divisions.

The sweeping back-and-forth drama of the first year's fighting in Korea was over, and the two National Guard divisions found themselves in a different kind of combat environment. As formal peace talks began in November 1951, U.N. and communist forces had settled themselves on either side of the 38th parallel. The National Guard divisions joined in a static warfare of entrenched positions and frequent combat patrols, punctuated by small-unit actions initiated by both sides.

Combat operations intensified once again in the spring of 1953, as both sides jockeyed for territory before a final border settlement. Both the 40th and 45th Infantry Divisions were still occupying their positions when the signing of an armistice at Panmunjon finally ended the fighting on July 27, 1953.

By this time, Army guardsmen who had arrived in South Korea during 1951 and early 1952 had returned home, their term of active federal service completed. But most National Guard units, now filled with draftees and enlistees, remained on active duty. Some stayed in Korea for several years, helping to monitor the fragile peace, but by 1955 almost all of the units federalized for the Korean War had been returned to state control.

Army National Guard Units that Served in Korea