IX Corps was a corps of the United States Army. For most of its operational history, IX Corps was headquartered in or around Japan and subordinate to US Army commands in the Far East. One of its battalions was the 424th Field Artillery Battalion (United States).

Created following World War I, the corps was not activated for use until just before World War II almost 20 years later. The corps spent most of World War II in charge of defenses on the West Coast of the United States, before moving to Hawaii and Leyte to plan and organize operations for US forces advancing across the Pacific. Following the end of the war, IX Corps participated in the occupation of mainland Japan.

The corps' only combat came in the Korean War. It is best known for its exploits as a senior command of the Eighth United States Army, commanding front line UN forces in numerous offensives and counteroffensives throughout the war. The corps served on the front lines for most of the conflict and took command of several combat divisions at a time. Following the end of the Korean War, IX Corps remained in Korea for several years until it was moved to Japan. The corps spent almost 40 years as an administrative command of the US Army forces there, overseeing administrative functions but no combat. It was finally inactivated and consolidated in 1994.

History

World War I IX Corps was formed from 25 to 29 November 1918 in Ligny-en-Barrois, France. It was demobilized in France on 5 May 1919.Organization and Insignia of the American Expeditionary Force IX Corps was subordinate to Second United States Army, and after moving its headquarters to Saint-Mihiel, and commanded forces along the armistice line between Jonville-en-Woëvre and Fresnes-en-Woëvre until its deactivation. Adelbert Cronkhite was the first corps commander, and William K. Naylor the first chief of staff.Order of Battle of the United States Land Forces in the World War, p. 343 Subsequent World War I commanders included Joseph E. Kuhn.

Post-World War I The IX Corps was demobilized on 5 May 1919 at Nogent-en-Bassigny, France, as Headquarters, IX Army Corps. It was reconstituted in the Organized Reserve on 29 July 1921 as the IX Corps (Headquarters Company concurrently constituted), allotted to the Ninth Corps Area, and assigned to the Third Army. An oddity was that only the VIII and IX Corps were considered direct lineal descendants of their World War I predecessors. Per General Order #29, Ninth Corps Area, the corps headquarters and headquarters company were activated on 7 October 1921 at the Presidio of San Francisco, California, with Regular Army and Organized Reserve personnel. It was assigned a shoulder sleeve insignia shortly thereafter. The corps headquarters was responsible for providing and planning administration, organization, supply, and training for army, corps, and other nondivisional Reserve units, less field and coast artillery, in the Ninth Corps Area. The headquarters was relieved from active duty on 9 May 1924 and all Regular Army personnel were relieved from assignment. Both the headquarters and the headquarters company remained active in the Reserve. Concurrently, the corps’ designated headquarters location for peacetime organization purposes was transferred from San Francisco to Seattle, Washington.

Reserve personnel assigned to the corps headquarters attended summer training at the Presidio of San Francisco. Also, individual staff officers attended active training from time to time with the Headquarters, 3rd Division at Fort Lewis, Washington. The headquarters conducted staff training with Headquarters, 30th Infantry, at the Presidio of San Francisco from 5-19 June 1927. The IX Corps headquarters and headquarters company were withdrawn from the Organized Reserve on 1 October 1933 and allotted to the Regular Army. Concurrently, the corps headquarters was partially activated with Regular Army personnel from Headquarters, Ninth Corps Area and Reserve personnel from the corps area at large. As part of the four army plan put into effect on 1 October 1933, the IX Corps was relieved from the Third Army and assigned to the Fourth Army. As "Regular Army Inactive" unit from 1933 to 1940, the corps headquarters was occasionally organized provisionally for short periods using its assigned Reserve officers and staff officers from Headquarters, Ninth Corps Area. These periods included several Fourth Army command post exercises in the 1930s and the Fourth Army maneuvers at Fort Lewis in August 1940.

World War II

The IX Corps headquarters was activated on 24 October 1940, less Reserve personnel, at Fort Lewis where it assumed command and control of the 3rd and 41st Divisions. The corps participated in the Fourth Army maneuvers during May–August 1941 at the Hunter Liggett Military Reservation, California. After the maneuvers, the corps headquarters returned to Fort Lewis where it was located on 7 December 1941. It immediately began training of combat units in preparation for deployment. One year later, IX Corps took command of the Camp Murray staging area in Washington, responsible for training Army National Guard forces in addition to its responsibilities training active duty and reserve units.

Following the Japanese attack on Pearl Harbor in late 1941, IX Corps was assigned to defensive duties on the West Coast of the United States, specifically the central and northern regions of the coast. The corps oversaw defenses on the West Coast for the majority of the war, but in 1944 it was moved to Fort McPherson, Georgia in preparation for deployment overseas.

Planning The corps trained at Fort McPherson in preparation for deployment to the Pacific Theater of Operations. On 25 September 1944, the corps closed headquarters at Fort McPherson and moved to Hawaii. When it arrived in Hawaii, IX Corps was put under the command of the Tenth United States Army. Under the Tenth Army, IX Corps was assigned two missions. In 1944, it was primarily concerned with formulating plans for an invasion of the coastal regions of Japanese-held China. Later in 1944 and early 1945, it was placed in charge of preparing the rest of the Tenth Army for movement to Okinawa in preparation for an invasion of the island, which was launched in April 1945.

When General of the Army Douglas MacArthur took overall command of Pacific Forces, IX Corps was moved to Leyte in the Philippine Islands and was assigned to the Sixth United States Army in July 1945. In Leyte, the corps was tasked with the planning of Operation Downfall, the invasion of mainland Japan, specifically the island of Kyushu. It was also tasked with planning occupation once Japan surrendered. IX Corps was assigned as one of four Corps under the command of the Sixth Army, with a strength of 14 divisions. With the 77th Infantry Division, the 81st Infantry Division and 98th Infantry Division, a force of 79,000 men, IX Corps would serve as the Sixth Army's reserve force during the initial invasion. Before the assault could be launched, Japan surrendered in August 1945, following the use of nuclear weapons on Hiroshima and Nagasaki.

Occupation Following the surrender, IX Corps was assigned command of occupation forces on the northern island of Hokkaidō. IX Corps transferred its headquarters in October 1945 to Sapporo for occupation duties. The next few years were a period during which the terms of the surrender were supervised and enforced; Japanese military installations and material were seized, troops were disarmed and discharged, and weapons of warfare disposed of. The duties of the occupation force included conversion of industry, repatriation of foreign nationals, and supervision of the complex features of all phases of Japanese government, economics, education, and industry.

As the occupation duties were accomplished, the occupation force continued to downsize as more troops returned home and their units were inactivated. By 1950, the Sixth Army had left Japan, and the occupation force was reduced to the Eighth United States Army commanding two corps and four under-strength divisions; the I Corps, commanding the 24th Infantry Division and 25th Infantry Division, and the IX Corps, commanding the 1st Cavalry Division and the 7th Infantry Division. IX Corps had been moved to Sendai as the occupation forces shifted as a result of the downsizing. As part of further downsizing, IX Corps was inactivated on 28 March 1950, and its command responsibilities were consolidated with other units.

Korean War Only a few months later, the Korean War began, and units from Japan began streaming into South Korea. The Eighth Army, taking charge of the conflict, requested the activation of three corps headquarters for its growing command of UN forces. IX Corps was activated on 10 August 1950 at Fort Sheridan, Illinois. Most of its personnel were transferred from the headquarters of the Fifth United States Army.

Pusan Perimeter

IX Corps arrived at the Pusan Perimeter in Korea on 22 September 1950, and became operational the next day when it took command of the 2nd Infantry Division and 25th Infantry Division. It took charge of the western flank of the perimeter, defending the Naktong River area against attacking North Korean Korean People's Army (KPA) units.

Amphibious landings at Inchon by X Corps hit KPA forces from behind, allowing I Corps to breakout from the Pusan perimeter starting on 16 September. Four days later I Corps troops began a general offensive northward against crumbling KPA opposition to establish contact with forces of the 7th Infantry Division driving southward from the beachhead. Major elements of the KPA were destroyed and cut off in this aggressive penetration; the link-up was effected south of Suwon on 26 September. The offensive was continued northwards, past Seoul, and across the 38th Parallel into North Korea on 1 October. The momentum of the attack was maintained, and the race to the North Korean capital, Pyongyang, ended on 19 October when elements of the Republic of Korea Army (ROK) 1st Infantry Division and the US 1st Cavalry Division captured the city. The advance continued, but against unexpectedly stiffening resistance. The Chinese People's Volunteer Army (PVA) entered the war on the side of North Korea, making their first attacks in late October. By the end of October the city of Chongju, from the Yalu River border of North Korea, had been captured. IX Corps advanced in the center of the Army, with I Corps along the west coast and X Corps operating independently further east. Commanders hoped the offensive would end the war "by Christmas."

Chinese intervention The UN forces renewed their offensive on 24 November before being stopped by the PVA Second Phase Offensive starting on 25 November with PVA forces penetrating the corps' rear from its exposed east flank. The 2nd Infantry Division, at the front of IX Corp's advance in Kunuri, was overwhelmed from all sides by PVA forces of the 40th Army Corps and elements from the 38th Army Corps on 29 November in the Battle of Kunuri. By 1 December, the division was almost completely destroyed; it lost virtually all of its heavy equipment and vehicles, as well as suffering 4,940 men killed or missing. The 25th Infantry Division, on its western flank, was also hit by overwhelming PVA forces of the 39th Army Corps, facing strong attacks and suffering heavy casualties and losses in equipment in the Battle of Ch'ongch'on River. However, it was spared the same losses as the 2nd Infantry Division by escaping across the Ch'ongch'on River. The Eighth Army suffered heavy casualties, ordering a complete withdrawal to the Imjin River, south of the 38th Parallel, having been destabilised by the overwhelming PVA forces. IX Corps retreated along the western coast to safety via Anju.